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

[Aaron Ballman 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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AaronBallman wrote:

They're tested:
```
// Smoke test that we catch keyword use in function parameters too.
void func(int private); // expected-warning {{identifier 'private' conflicts with a C++ keyword}} \
                           cxx-error {{invalid parameter name: 'private' is a keyword}}
```
around line 192

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