[clang] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

[Vlad Serebrennikov via cfe-commits]

================
@@ -584,8 +584,8 @@ namespace dr647 { // dr647: 3.1
   struct C {
     constexpr C(NonLiteral);
     constexpr C(NonLiteral, int) {}
-    // since-cxx11-error at -1 {{constexpr constructor's 1st parameter type 'NonLiteral' is not a literal type}}
-    //   since-cxx11-note@#dr647-NonLiteral {{'NonLiteral' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors}}
+    // cxx11-20-error at -1 {{constexpr constructor with 1st non-literal parameter type 'NonLiteral' is a C++23 extension}}
+    // cxx11-20-note@#dr647-NonLiteral {{'NonLiteral' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors}}
----------------
Endilll wrote:

```suggestion
    //   cxx11-20-note@#dr647-NonLiteral {{'NonLiteral' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors}}
```

https://github.com/llvm/llvm-project/pull/77753