Clang: Remove -Wperf-constraint-implies-noexcept from -Wall. (llvm#167540)

In adopting `[[clang::nonblocking]]` there's been some user confusion.
Changes to address `-Wfunction-effects` warnings are often pure
annotation, with no runtime effect. Changes to avoid
`-Wperf-constraint-implies-noexcept` warnings are risky: adding
`noexcept` creates a new potential for the program to crash. In
retrospect, `-Wperf-constraint-implies-noexcept` shouldn't have been
made part of `-Wall`.

---------

Co-authored-by: Doug Wyatt <[email protected]>

Author: dougsonos

clang/docs/ReleaseNotes.rst

@@ -359,6 +359,8 @@ 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.
+- Remove ``-Wperf-constraint-implies-noexcept`` from ``-Wall``. This warning is somewhat nit-picky and
+  attempts to resolve it, by adding ``noexcept``, can create new ways for programs to crash. (#GH167540)
 - Implemented diagnostics when retrieving the tuple size for types where its specialization of `std::tuple_size`
   produces an invalid size (either negative or greater than the implementation limit). (#GH159563)
 - Fixed fix-it hint for fold expressions. Clang now correctly places the suggested right

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1314,7 +1314,7 @@ def Consumed       : DiagGroup<"consumed">;
 // DefaultIgnore in addition to putting it here.
 def All : DiagGroup<"all", [Most, Parentheses, Switch, SwitchBool,
                             MisleadingIndentation, PackedNonPod,
-                            VLACxxExtension, PerfConstraintImpliesNoexcept]>;
+                            VLACxxExtension]>;
 
 // Warnings that should be in clang-cl /w4.
 def : DiagGroup<"CL4", [All, Extra]>;

clang/test/Misc/warning-wall.c

@@ -109,6 +109,5 @@ CHECK-NEXT:  -Wmisleading-indentation
 CHECK-NEXT:  -Wpacked-non-pod
 CHECK-NEXT:  -Wvla-cxx-extension
 CHECK-NEXT:    -Wvla-extension-static-assert
-CHECK-NEXT:  -Wperf-constraint-implies-noexcept
 
 CHECK-NOT:-W