[C++20] [Modules] Allow export redeclarations within language linkage

Close #98583

Currently, clang will reject the following code:

```
export module mod;
extern "C++" void func();
export extern "C++" {
    void func();
}
```

while both MSVC and GCC accept it. Although clang's behavior matches the
current wording, from the discussion, the consensus is that we should
accept the above example from the intention. Since the intention to not
allow export redeclaration which is not exported is to make the linkage
clear. But it doesn't matter with the declarations within global module.

Author: ChuanqiXu9

clang/lib/Sema/SemaDecl.cpp

@@ -1676,6 +1676,15 @@ bool Sema::CheckRedeclarationExported(NamedDecl *New, NamedDecl *Old) {
   if (IsOldExported)
     return false;
 
+  // If the Old declaration are not attached to named modules
+  // and the New declaration are attached to global module.
+  // It should be fine to allow the export since it doesn't change
+  // the linkage of declarations. See
+  // https://github.com/llvm/llvm-project/issues/98583 for details.
+  if (!Old->isInNamedModule() && New->getOwningModule() &&
+      New->getOwningModule()->isImplicitGlobalModule())
+    return false;
+
   assert(IsNewExported);
 
   auto Lk = Old->getFormalLinkage();

clang/test/Modules/export-redecl-in-language-linkage.cppm

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -std=c++20 %s -verify -fsyntax-only
+
+// expected-no-diagnostics
+export module mod;
+extern "C++" void func();
+export extern "C++" {
+    void func();
+}