[clang] [modules] Add err_main_in_named_module (#146247)

Close https://github.com/llvm/llvm-project/issues/146229

As the issue said, main shouldn't be in any modules.

new diagnostic output:
```
/my/code/directory/main.cpp:3:1: warning: 'main' should not be attached to a named module; consider adding C++ language linkage [-Wmain]
    3 | int main() {
      | ^
      | extern "C++" 
1 warning generated.
```

clang/docs/ReleaseNotes.rst

@@ -650,6 +650,9 @@ Improvements to Clang's diagnostics
   #GH69470, #GH59391, #GH58172, #GH46215, #GH45915, #GH45891, #GH44490,
   #GH36703, #GH32903, #GH23312, #GH69874.
 
+- A warning is now emitted when ``main`` is attached to a named module,
+  which can be turned off with ``-Wno-main-attached-to-named-module``. (#GH146247)
+
 - Clang now avoids issuing `-Wreturn-type` warnings in some cases where
   the final statement of a non-void function is a `throw` expression, or
   a call to a function that is trivially known to always throw (i.e., its

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1062,6 +1062,10 @@ def err_constexpr_main : Error<
   "'main' is not allowed to be declared %select{constexpr|consteval}0">;
 def err_deleted_main : Error<"'main' is not allowed to be deleted">;
 def err_mainlike_template_decl : Error<"%0 cannot be a template">;
+def warn_main_in_named_module
+    : ExtWarn<"'main' should not be attached to a named module; consider "
+              "adding C++ language linkage">,
+      InGroup<DiagGroup<"main-attached-to-named-module">>;
 def err_main_returns_nonint : Error<"'main' must return 'int'">;
 def ext_main_returns_nonint : ExtWarn<"return type of 'main' is not 'int'">,
     InGroup<MainReturnType>;

clang/lib/Sema/SemaDecl.cpp

@@ -12489,6 +12489,14 @@ void Sema::CheckMain(FunctionDecl *FD, const DeclSpec &DS) {
                                 : FixItHint());
       FD->setInvalidDecl(true);
     }
+
+    // In C++ [basic.start.main]p3, it is said a program attaching main to a
+    // named module is ill-formed.
+    if (FD->isInNamedModule()) {
+      const SourceLocation start = FD->getTypeSpecStartLoc();
+      Diag(start, diag::warn_main_in_named_module)
+          << FixItHint::CreateInsertion(start, "extern \"C++\" ", true);
+    }
   }
 
   // Treat protoless main() as nullary.

clang/test/SemaCXX/modules.cppm

@@ -68,6 +68,8 @@ int n;
 //--- test3.cpp
 export module bar;
 
+int main() {} // expected-warning {{'main' should not be attached to a named module; consider adding C++ language linkage}}
+
 static int m;
 
 int n;