[clang] [Clang] disallow constexpr with auto and explicit type in C23 (PR #163469)

[Mariya Podchishchaeva via cfe-commits]

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@@ -62,6 +62,8 @@ auto basic_usage(auto auto) {   // c23-error {{'auto' not allowed in function pr
 
   int auto_cxx_decl = auto(0);  // expected-error {{expected expression}}
 
+  constexpr auto int x = 0; // c23-error {{cannot combine with previous 'auto' declaration specifier}} \
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Fznamznon wrote:

Can we add a test (if not yet existent) that does
```
constexpr int auto y = 9;
```

https://github.com/llvm/llvm-project/pull/163469