[clang] [Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (PR #95474)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

@kripken somewhat reduced:

```cpp
struct MyStruct {
    double m0{-9999.0};
    int m1{-12345};
};

constexpr MyStruct default_val;

auto compute_area(double l, const MyStruct &val = default_val) -> double {
    if (val.m1 == 1)
      return 2.0;
    return 0;
}


#include <cassert>


auto main() -> int {
    MyStruct in_val{.m0 = 2.0, .m1 = 1};
    double val = compute_area(1.0, in_val);
    assert(val == 2.0);
}
```



https://github.com/llvm/llvm-project/pull/95474