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

[Amy Huang via cfe-commits]

amykhuang wrote:

Hi, this is causing failures for Chrome, building with c++20:

```
$ cat t.cc
template <typename Type> class StrongAlias {
  friend constexpr bool operator==(const StrongAlias &, const StrongAlias &) = default;
  Type value;
};

class Token {};
bool operator==(const Token&, const Token&);

StrongAlias<Token> a;

$ clang -std=c++20 t.cc
t.cc:2:3: error: defaulted definition of equality comparison operator cannot be declared constexpr because it invokes a non-constexpr comparison function 
    2 |   friend constexpr bool operator==(const StrongAlias &, const StrongAlias &) = default;
      |   ^
t.cc:9:20: note: in instantiation of template class 'StrongAlias<Token>' requested here
    9 | StrongAlias<Token> a;
      |                    ^
t.cc:3:8: note: non-constexpr comparison function would be used to compare member 'value'
    3 |   Type value;
      |        ^
t.cc:7:6: note: non-constexpr comparison function declared here
    7 | bool operator==(const Token&, const Token&);
      |      ^
1 error generated.
```

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