[clang] [C] Diagnose declarations hidden in C++ (PR #137368)

[Erich Keane via cfe-commits]

================
@@ -1428,6 +1428,17 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
   return false;
 }
 
+bool DeclContext::LexicallyEncloses(const DeclContext *DC) const {
+  if (getPrimaryContext() != this)
+    return getPrimaryContext()->LexicallyEncloses(DC);
+
+  for (; DC; DC = DC->getLexicalParent())
+    if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
----------------
erichkeane wrote:

```suggestion
    if (!isa<LinkageSpecDecl, ExportDecl>(DC) &&
```

That said... Perhaps we should just do a 

```
if (isa<LinkageSpecDecl, ExportDecl>(*this)) return false;
```
??  Since thats the only way the equivilence would work? 



https://github.com/llvm/llvm-project/pull/137368