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

[Aaron Ballman via cfe-commits]

================
@@ -111,3 +111,12 @@ namespace non_trivially_copyable {
   _Atomic S s;  // expected-error {{_Atomic cannot be applied to type 'S' which is not trivially copyable}} \
                 // expected-warning {{'_Atomic' is a C11 extension}}
 }
+
+namespace static_cast_crash {
+  struct S { char a; };
+  void static_cast_non_atomic_to_atomic() {
+    _Atomic struct S a; // expected-warning {{'_Atomic' is a C11 extension}}
+    a = static_cast<_Atomic(struct S)>(S()); // expected-warning {{'_Atomic' is a C11 extension}}
----------------
AaronBallman wrote:

Another interesting test case:
```
struct S {
  operator float() const;
  int a;
};

struct T {
  operator _Atomic(S)() const;
};

void static_cast_non_atomic_to_atomic() {
  _Atomic struct S s;
  s = static_cast<_Atomic(S)>(T{});
}
```
This fails today but should succeed with your patch. But then there's the sadder test case:
```
struct S {
  operator float() const;
  int a;
};

struct T {
  operator _Atomic(S)() const;
};

void static_cast_non_atomic_to_atomic() {
  float f;
  f = static_cast<float>(static_cast<_Atomic(S)>(T{}));
}
```
This *won't* work because the atomic is not being stripped from the cast and you cannot access members of an atomic object (so no way to call the conversion operator).

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