[clang] [Clang] strengthen checks for 'main' function to meet [basic.start.main] p3 requirements (PR #101853)

[Mital Ashok via cfe-commits]

================
@@ -12210,7 +12220,18 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
   return Redeclaration;
 }
 
-void Sema::CheckMain(FunctionDecl* FD, const DeclSpec& DS) {
+void Sema::CheckMain(FunctionDecl *FD, const DeclSpec &DS) {
+  // [basic.start.main]p3
+  //    The main function shall not be declared with a linkage-specification.
+  if (FD->isExternCContext() ||
+      (FD->isExternCXXContext() &&
+       FD->getDeclContext()->getRedeclContext()->isTranslationUnit())) {
+    Diag(FD->getLocation(), diag::ext_main_invalid_linkage_specification)
+        << FD->getLanguageLinkage();
+    FD->setInvalidDecl();
+    return;
----------------
MitalAshok wrote:

```suggestion
```

@alexfh I think these lines are probably what's causing the crash (if it's allowed as an extension, it should not be marked invalid, and we need to diagnose/check the other stuff if we have `-Wno-main`).

Either way, could you open a new GitHub issue for visibility?


https://github.com/llvm/llvm-project/pull/101853