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

[Aaron Ballman via cfe-commits]

================
@@ -2487,6 +2477,26 @@ static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
   return true;
 }
 
+static bool CheckConstexprMissingReturn(Sema &SemaRef,
+                                        const FunctionDecl *Dcl) {
+  bool IsVoidOrDependentType = Dcl->getReturnType()->isVoidType() ||
+                               Dcl->getReturnType()->isDependentType();
+
+  if (SemaRef.getLangOpts().CPlusPlus23 && !IsVoidOrDependentType)
+    return true;
+
+  // C++1y doesn't require constexpr functions to contain a 'return'
----------------
AaronBallman wrote:

```suggestion
  // C++14 doesn't require constexpr functions to contain a 'return'
```

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