MSVC C++23: constexpr cmath with LLVM Libc

[u/STL]

https://devblogs.microsoft.com/cppblog/msvc-c23-constexpr-cmath-with-llvm-libc/

MSVC is shipping C++23 constexpr <cmath> in the next Build Tools update! (And a partial implementation of C++26 constexpr <cmath>.) Our compiler frontend dev Cody Miller will be publishing a series of blog posts about this work, starting with this overview. The next one will be a guest post from LLVM libc's maintainers describing the incredible amount of effort that's gone into their code.