[clang] [C23] Implement N3018: The constexpr specifier for object definitions (PR #73099)

[Aaron Ballman via cfe-commits]

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@@ -1382,6 +1382,20 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
       ThreadStorageClassSpec = TSCS_unspecified;
       ThreadStorageClassSpecLoc = SourceLocation();
     }
+    if (S.getLangOpts().C23 &&
+        getConstexprSpecifier() == ConstexprSpecKind::Constexpr) {
+      S.Diag(ConstexprLoc, diag::err_invalid_decl_spec_combination)
+          << DeclSpec::getSpecifierName(getThreadStorageClassSpec())
+          << SourceRange(getThreadStorageClassSpecLoc());
+    }
+  }
+
+  if (S.getLangOpts().C23 &&
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AaronBallman wrote:

I think we need something similar here for mixing `typedef` and `constexpr`. A potential test case would be:
```
typedef constexpr int Foo;
constexpr typedef int Bar;
```

https://github.com/llvm/llvm-project/pull/73099