[clang] [Clang][C++23] update constexpr diagnostics for missing return statements per P2448 (PR #94123)

[Oleksandr T. via cfe-commits]

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@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -Wno-return-type -std=c++23 -fsyntax-only -verify %s
+// expected-no-diagnostics
+constexpr int f() { }
+static_assert(__is_same(decltype([] constexpr -> int { }( )), int));
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a-tarasyuk wrote:

@AaronBallman Should a new option (`-Winvalid-constexpr`) be added to control warnings for this case? Is that what you're referring to?

https://github.com/llvm/llvm-project/pull/94123