[clang] [Clang] fix crash by avoiding invalidation of extern main declaration during strictness checks (PR #104594)

[Alexander Kornienko via cfe-commits]

================
@@ -122,6 +119,26 @@ namespace ns2 {
   extern "C++" void main() {} // ok
 }
 
+#elif TEST14
+extern "C" {
+  int main(); // expected-warning {{'main' should not be 'extern "C"'}}
+}
+
+extern "C" int main(); // expected-warning {{'main' should not be 'extern "C"'}}
+
+#elif TEST15
+extern "C" __attribute__((visibility("default"))) __attribute__((weak))
----------------
alexfh wrote:

nit: This can be simplified a bit:
```
extern "C" __attribute__((visibility("default"))) __attribute__((weak))
int main();

unsigned long g() {
  return reinterpret_cast<unsigned long>(&main);
}
```
https://gcc.godbolt.org/z/j5crjfebq

https://github.com/llvm/llvm-project/pull/104594