[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:

I'm also not yet convinced this is correct (or incorrect either, this is complicated stuff). Consider this example:
```
struct S {
  int get() const { return a; }
  int a;
};

void static_cast_non_atomic_to_atomic() {
  int i;
  struct S s;
  i = static_cast<_Atomic S>(s).get();
}
```
https://godbolt.org/z/s4MdYbrn9

The original code would have stripped the `_Atomic` qualifier here, so what's causing the "can't access an atomic object" diagnostics?

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