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

[Mital Ashok via cfe-commits]

================
@@ -8052,10 +8061,7 @@ NamedDecl *Sema::ActOnVariableDeclarator(
   }
 
   // Special handling of variable named 'main'.
-  if (Name.getAsIdentifierInfo() && Name.getAsIdentifierInfo()->isStr("main") &&
-      NewVD->getDeclContext()->getRedeclContext()->isTranslationUnit() &&
-      !getLangOpts().Freestanding && !NewVD->getDescribedVarTemplate()) {
-
+  if (!getLangOpts().Freestanding && isMainVar(Name, NewVD)) {
     // C++ [basic.start.main]p3
     // A program that declares a variable main at global scope is ill-formed.
----------------
MitalAshok wrote:

```suggestion
    // C++ [basic.start.main]p3:
    //   A program that declares
    //    - a variable main at global scope, or
    //    - an entity named main with C language linkage (in any namespace)
    //   is ill-formed.
```

To facilitate the diagnostic showing whether its a global variable or a C linkage variable, you could make `isMainVar` return an enum like `IMV_NotMain, IMV_GlobalMain, IMV_CLinkageMain`. Or you could inline the `isMainVar` function into here again since there would be three options now. This quote from the standard should be good enough documentation

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