[clang] [C] Diagnose use of C++ keywords in C (PR #137234)

[Erich Keane via cfe-commits]

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@@ -10416,6 +10447,15 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
   // Finally, we know we have the right number of parameters, install them.
   NewFD->setParams(Params);
 
+  // If this declarator is a declaration and not a definition, its parameters
+  // will not be pushed onto a scope chain. That means we will not issue any
+  // reserved identifier warnings for the declaration, but we will for the
+  // definition. Handle those here.
+  if (!Params.empty() && !D.isFunctionDefinition()) {
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erichkeane wrote:

I suspect THIS is where your slowdown is going to come from, and I see no tests for this (that is, function parameters).

ALSO, `!Params.empty()` isnt' really necessary, the `for` loop will do that for you.

https://github.com/llvm/llvm-project/pull/137234