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

[Aaron Ballman via cfe-commits]

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@@ -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) &&
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AaronBallman wrote:

This code was lifted from `DeclContext::Encloses()`, so the cleanup is easy because it doesn't change logic between the two functions. I'll clean that up, but other changes should be done in a follow-up so they're done consistently (if done at all). WDYT?

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