[libc++] Assume that compilers support extended constexpr in C++14 mode

We don't support any compiler that doesn't support C++14 constexpr when
compiling in C++14 mode anymore, so we can just assume that we have C++14
extended constexpr when compiling in C++14 mode. This allows us to remove
some workarounds for older compilers.

Differential Revision: https://reviews.llvm.org/D108638