[PATCH 1/7] [clang] Improve nested name specifier AST representation

This is a major change on how we represent nested name qualifications in the AST.

* The nested name specifier itself and how it's stored is changed. The prefixes for types are handled within the type hierarchy, which makes canonicalization for them super cheap, no memory allocation required. Also translating a type into nested name specifier form becomes a no-op. An identifier is stored as a DependentNameType. The nested name specifier gains a lightweight handle class, to be used instead of passing around pointers, which is similar to what is implemented for TemplateName. There is still one free bit available, and this handle can be used within a PointerUnion and PointerIntPair, which should keep bit-packing aficionados happy.
* The ElaboratedType node is removed, all type nodes in which it could previously apply to can now store the elaborated keyword and name qualifier, tail allocating when present.
* TagTypes can now point to the exact declaration found when producing these, as opposed to the previous situation of there only existing one TagType per entity. This increases the amount of type sugar retained, and can have several applications, for example in tracking module ownership, and other tools which care about source file origins, such as IWYU. These TagTypes are lazily allocated, in order to limit the increase in AST size.

This patch offers a great performance benefit.

It greatly improves compilation time for [stdexec](https://github.com/NVIDIA/stdexec). For one datapoint, for `test_on2.cpp` in that project, which is the slowest compiling test, this patch improves `-c` compilation time by about 7.2%, with the `-fsyntax-only` improvement being at ~12%.

This has great results on compile-time-tracker as well:
![image](https://github.com/user-attachments/assets/700dce98-2cab-4aa8-97d1-b038c0bee831)

This patch also further enables other optimziations in the future, and will reduce the performance impact of template specialization resugaring when that lands.

It has some other miscelaneous drive-by fixes.

About the review: Yes the patch is huge, sorry about that. Part of the reason is that I started by the nested name specifier part, before the ElaboratedType part, but that had a huge performance downside, as ElaboratedType is a big performance hog. I didn't have the steam to go back and change the patch after the fact.

There is also a lot of internal API changes, and it made sense to remove ElaboratedType in one go, versus removing it from one type at a time, as that would present much more churn to the users. Also, the nested name specifier having a different API avoids missing changes related to how prefixes work now, which could make existing code compile but not work.

How to review: The important changes are all in `clang/include/clang/AST` and `clang/lib/AST`, with also important changes in `clang/lib/Sema/TreeTransform.h`.

The rest and bulk of the changes are mostly consequences of the changes in API.

PS: TagType::getDecl is renamed to `getOriginalDecl` in this patch, just for easier to rebasing. I plan to rename it back after this lands.

Fixes #136624
Fixes #147000

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -70,12 +70,16 @@ ABI Changes in This Version
 
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
+- How nested name specifiers are dumped and printed changes, keeping track of clang AST changes.
 
 Clang Frontend Potentially Breaking Changes
 -------------------------------------------
 
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
+- TypeKind ``ELABORATED`` is not used anymore, per clang AST changes removing
+  ElaboratedTypes. The value becomes unused, and all the existing users should
+  expect the former underlying type to be reported instead.
 
 What's New in Clang |release|?
 ==============================
@@ -111,13 +115,13 @@ Non-comprehensive list of changes in this release
 -------------------------------------------------
 - Added ``__builtin_elementwise_minnumnum`` and ``__builtin_elementwise_maxnumnum``.
 
-- Trapping UBSan (e.g. ``-fsanitize-trap=undefined``) now emits a string describing the reason for 
-  trapping into the generated debug info. This feature allows debuggers (e.g. LLDB) to display 
-  the reason for trapping if the trap is reached. The string is currently encoded in the debug 
-  info as an artificial frame that claims to be inlined at the trap location. The function used 
-  for the artificial frame is an artificial function whose name encodes the reason for trapping. 
-  The encoding used is currently the same as ``__builtin_verbose_trap`` but might change in the future. 
-  This feature is enabled by default but can be disabled by compiling with 
+- Trapping UBSan (e.g. ``-fsanitize-trap=undefined``) now emits a string describing the reason for
+  trapping into the generated debug info. This feature allows debuggers (e.g. LLDB) to display
+  the reason for trapping if the trap is reached. The string is currently encoded in the debug
+  info as an artificial frame that claims to be inlined at the trap location. The function used
+  for the artificial frame is an artificial function whose name encodes the reason for trapping.
+  The encoding used is currently the same as ``__builtin_verbose_trap`` but might change in the future.
+  This feature is enabled by default but can be disabled by compiling with
   ``-fno-sanitize-annotate-debug-info-traps``.
 
 New Compiler Flags
@@ -143,7 +147,7 @@ Improvements to Clang's diagnostics
   Moved the warning for a missing (though implied) attribute on a redeclaration into this group.
   Added a new warning in this group for the case where the attribute is missing/implicit on
   an override of a virtual method.
-- Fixed fix-it hint for fold expressions. Clang now correctly places the suggested right 
+- Fixed fix-it hint for fold expressions. Clang now correctly places the suggested right
   parenthesis when diagnosing malformed fold expressions. (#GH151787)
 
 Improvements to Clang's time-trace
@@ -187,6 +191,9 @@ Bug Fixes to C++ Support
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
+- Fix incorrect name qualifiers applied to alias CTAD. (#GH136624)
+- Fixed ElaboratedTypes appearing within NestedNameSpecifier, which was not a
+  legal representation. This is fixed because ElaboratedTypes don't exist anymore. (#GH43179) (#GH68670) (#GH92757)
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^
@@ -256,6 +263,8 @@ Fixed Point Support in Clang
 
 AST Matchers
 ------------
+- Removed elaboratedType matchers, and related nested name specifier changes,
+  following the corresponding changes in the clang AST.
 - Ensure ``hasBitWidth`` doesn't crash on bit widths that are dependent on template
   parameters.
 
@@ -280,7 +289,7 @@ New features
 
 Crash and bug fixes
 ^^^^^^^^^^^^^^^^^^^
-- Fixed a crash in the static analyzer that when the expression in an 
+- Fixed a crash in the static analyzer that when the expression in an
   ``[[assume(expr)]]`` attribute was enclosed in parentheses.  (#GH151529)
 
 Improvements