[libc++] P2641R4: Checking if a `union` alternative is active (`std::is_within_lifetime`) (#165243)

<https://wg21.link/P2641R4>

Implements the C++26 function in `<type_traits>` [meta.const.eval] (and
the corresponding feature test macro `__cpp_lib_is_within_lifetime`)

```c++
template<class T>
  consteval bool is_within_lifetime(const T*) noexcept;
```

This is done with the `__builtin_is_within_lifetime` builtin added to
Clang 20 by #91895 / 2a07509c8d3c8b5b2c88e4f73dde0071bf506870. This is
not (currently) available with GCC.

This implementation has provisions for LWG4138
<https://cplusplus.github.io/LWG/issue4138> where it is ill-formed to
instantiate `is_within_lifetime<T>` with a function type `T`.

Closes #105381

Co-authored-by: Mital Ashok <mital@mitalashok.co.uk>

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