[ADT] Replace STLForwardCompat.h's C++17 equivalents

STLForwardCompat.h defines several utilities and type traits to mimic that of
the ones in the C++17 standard library. Now that LLVM is built with the C++17
standards mode, remove use of these equivalents in favor of the ones from the
standard library.

Differential Revision: https://reviews.llvm.org/D131717