[libcxx] [clang] [Clang][libc++] Implement __is_nothrow_convertible and use it in libc++ (PR #80436)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@philnik777 https://lab.llvm.org/buildbot/#/builders/46/builds/63116 

https://github.com/llvm/llvm-project/pull/80436