[clang] Reapply "[Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (#78274)" (PR #79683)

[via cfe-commits]

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@@ -6535,6 +6543,12 @@ NamedDecl *Sema::HandleDeclarator(Scope *S, Declarator &D,
   if (getLangOpts().CPlusPlus)
     CheckExtraCXXDefaultArguments(D);
 
+  // The scope passed in may not be a decl scope.  Zip up the scope tree until
+  // we find one that is.
+  while ((S->getFlags() & Scope::DeclScope) == 0 ||
+         (S->getFlags() & Scope::TemplateParamScope) != 0)
+    S = S->getParent();
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cor3ntin wrote:

We do that in at least 3 places (search for `(S->getFlags() & Scope::DeclScope)` )
Can we add a `Scope::getInnermostEnclosingDeclarativeScope()` or something like that ?

https://github.com/llvm/llvm-project/pull/79683