[llvm] minor fixes for clang-cl Windows DLL build (#144386)

## Purpose

This patch makes a minor changes to LLVM and Clang so that LLVM can be
built as a Windows DLL with `clang-cl`. These changes were not required
for building a Windows DLL with MSVC.

## Background

The Windows DLL effort is tracked in #109483. Additional context is
provided in [this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).

## Overview
Specific changes made in this patch:
- Remove `constexpr` fields that reference DLL exported symbols. These
symbols cannot be resolved at compile time when building a Windows DLL
using `clang-cl`, so they cannot be `constexpr`. Instead, they are made
`const` and initialized in the implementation file rather than at
declaration in the header.
- Annotate symbols now defined out-of-line with `LLVM_ABI` so they are
exported when building as a shared library.
- Explicitly add default copy assignment operator for `ELFFile` to
resolve a compiler warning.

## Validation

Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:

- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang

Author: andrurogerz

clang/lib/StaticAnalyzer/Core/Z3CrosscheckVisitor.cpp

@@ -92,7 +92,7 @@ void Z3CrosscheckVisitor::finalizeVisitor(BugReporterContext &BRC,
   };
 
   auto AttemptOnce = [&](const llvm::SMTSolverRef &Solver) -> Z3Result {
-    constexpr auto getCurrentTime = llvm::TimeRecord::getCurrentTime;
+    auto getCurrentTime = llvm::TimeRecord::getCurrentTime;
     unsigned InitialRLimit = GetUsedRLimit(Solver);
     double Start = getCurrentTime(/*Start=*/true).getWallTime();
     std::optional<bool> IsSAT = Solver->check();

llvm/include/llvm/BinaryFormat/Dwarf.h

@@ -1191,32 +1191,32 @@ template <typename Enum> struct EnumTraits : public std::false_type {};
 
 template <> struct EnumTraits<Attribute> : public std::true_type {
   static constexpr char Type[3] = "AT";
-  static constexpr StringRef (*StringFn)(unsigned) = &AttributeString;
+  LLVM_ABI static StringRef (*const StringFn)(unsigned);
 };
 
 template <> struct EnumTraits<Form> : public std::true_type {
   static constexpr char Type[5] = "FORM";
-  static constexpr StringRef (*StringFn)(unsigned) = &FormEncodingString;
+  LLVM_ABI static StringRef (*const StringFn)(unsigned);
 };
 
 template <> struct EnumTraits<Index> : public std::true_type {
   static constexpr char Type[4] = "IDX";
-  static constexpr StringRef (*StringFn)(unsigned) = &IndexString;
+  LLVM_ABI static StringRef (*const StringFn)(unsigned);
 };
 
 template <> struct EnumTraits<Tag> : public std::true_type {
   static constexpr char Type[4] = "TAG";
-  static constexpr StringRef (*StringFn)(unsigned) = &TagString;
+  LLVM_ABI static StringRef (*const StringFn)(unsigned);
 };
 
 template <> struct EnumTraits<LineNumberOps> : public std::true_type {
   static constexpr char Type[4] = "LNS";
-  static constexpr StringRef (*StringFn)(unsigned) = &LNStandardString;
+  LLVM_ABI static StringRef (*const StringFn)(unsigned);
 };
 
 template <> struct EnumTraits<LocationAtom> : public std::true_type {
   static constexpr char Type[3] = "OP";
-  static constexpr StringRef (*StringFn)(unsigned) = &OperationEncodingString;
+  LLVM_ABI static StringRef (*const StringFn)(unsigned);
 };
 
 inline uint64_t computeTombstoneAddress(uint8_t AddressByteSize) {

llvm/include/llvm/Object/ELF.h

@@ -256,8 +256,10 @@ class ELFFile {
 public:
   LLVM_ELF_IMPORT_TYPES_ELFT(ELFT)
 
-  // Default ctor required to instantiate the template for DLL export.
+  // Default ctor and copy assignment operator required to instantiate the
+  // template for DLL export.
   ELFFile(const ELFFile &) = default;
+  ELFFile &operator=(const ELFFile &) = default;
 
   // This is a callback that can be passed to a number of functions.
   // It can be used to ignore non-critical errors (warnings), which is

llvm/lib/BinaryFormat/Dwarf.cpp

@@ -911,6 +911,18 @@ StringRef llvm::dwarf::RLEString(unsigned RLE) {
   }
 }
 
+StringRef (*const llvm::dwarf::EnumTraits<Tag>::StringFn)(unsigned) = TagString;
+StringRef (*const llvm::dwarf::EnumTraits<Attribute>::StringFn)(unsigned) =
+    AttributeString;
+StringRef (*const llvm::dwarf::EnumTraits<Form>::StringFn)(unsigned) =
+    FormEncodingString;
+StringRef (*const llvm::dwarf::EnumTraits<LocationAtom>::StringFn)(unsigned) =
+    OperationEncodingString;
+StringRef (*const llvm::dwarf::EnumTraits<LineNumberOps>::StringFn)(unsigned) =
+    LNStandardString;
+StringRef (*const llvm::dwarf::EnumTraits<Index>::StringFn)(unsigned) =
+    IndexString;
+
 constexpr char llvm::dwarf::EnumTraits<Attribute>::Type[];
 constexpr char llvm::dwarf::EnumTraits<Form>::Type[];
 constexpr char llvm::dwarf::EnumTraits<Index>::Type[];