[clang] [clang][SemaCXX] Fix crash when using static_cast to _Atomic types in C++ (PR #207616)

[Aaron Ballman via cfe-commits]

================
@@ -78,7 +78,13 @@ namespace {
       // the qualifier.
       if (!S.Context.getLangOpts().ObjC && !DestType->isRecordType() &&
           !DestType->isArrayType() && !DestType.getPointerAuth()) {
-        DestType = DestType.getAtomicUnqualifiedType();
+        if (S.Context.getLangOpts().CPlusPlus) {
+          // Note that in C++, _Atomic(T) is a distinct type, not a
+          // cv-qualifier, so it is not stripped.
+          DestType = DestType.getUnqualifiedType();
+        } else {
+          DestType = DestType.getAtomicUnqualifiedType();
+        }
----------------
AaronBallman wrote:

`_Atomic` isn't in C++, so the C++ standard doesn't mandate anything here. (For example, we intentionally don't strip qualifications for ptrauth types either; extensions get to deviate from the standard but `_Atomic` is a bit of a special case because it is covered by the C standard.)

 My question was more prosaic though -- if we're incorrectly stripping the atomic qualification here today but managing to get the expected diagnostic about atomics, something elsewhere likely also needs to be adjusted because something was picking the atomic qualification back up. So I was looking for more details about what's going on there in the compiler.

It turns out that we're incorrect in C too: https://godbolt.org/z/rP8dn9a5M because `typeof` should be giving you the type with all qualifications, but the atomic qualifier was supposed to be dropped by the cast; this could be related to whatever is picking those qualifications back up in C++ mode.

https://github.com/llvm/llvm-project/pull/207616