[clang] [Clang] Fix crash for _Countof(void) by correcting extension handling (PR #181169)

[Aaron Ballman via cfe-commits]

================
@@ -35,6 +35,8 @@ void test_semantic_failures() {
   int non_array;
   (void)_Countof non_array;  // expected-error {{'_Countof' requires an argument of array type; 'int' invalid}}  
   (void)_Countof(int);       // expected-error {{'_Countof' requires an argument of array type; 'int' invalid}}  
+  (void)_Countof(void);      // expected-error {{invalid application of '_Countof' to an incomplete type 'void'}}
+  int arr2[_Countof(void)];  // expected-error {{invalid application of '_Countof' to an incomplete type 'void'}}
----------------
AaronBallman wrote:

One more test we should consider is with an expression operand rather than a type operand:
```
void *vp = &non_array;
(void)_Countof(*vp); // error, applied to incomplete type void
```

https://github.com/llvm/llvm-project/pull/181169