[libc++] Fix ambiguous call in {ranges, std}::find

[winner245]

This PR fixes an ambiguous call encountered when using the std::ranges::find or std::find algorithms with vector<bool> and a custom-sized allocator.

The root cause is that the internal bitwise arithmetic of the find algorithms for small integral types exhibits integral promotions, resulting in no single best viable function among the three candidates (__libcpp_ctz(unsigned), __libcpp_ctz(unsigned long), and __libcpp_ctz(unsigned long long)), causing an ambiguous call error.

To fix this ambiguity, we utilize a dispatcher function __countr_zero to directs calls to the right overload of __libcpp_ctz. Additionally, we have enhanced the internal implementation of __countr_zero, which was constexpr since C++14 but did not utilize constexpr if for C++17 and later. The improvements include:

• Making __countr_zero constexpr starting from C++11
• Eliminating runtime if overhead by employing constexpr if for C++17 and above, while using SFINAE-based overloading for standards prior to C++17.

Fixes #122528.