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

[Aaron Ballman via cfe-commits]

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@@ -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
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AaronBallman wrote:

Atomic is a distinct type in both languages; in C the specification is weird in that `_Atomic(T)` is a type *specifier* while `_Atomic T` is a type *qualifier* but the semantics of the type itself is still that it's an `AtomicType` which is distinct from the underlying type.

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