As a follow-up of e8578968f6 which replaced the
callers to use the C++17 equivalents, remove the equivalents from
`STLForwardCompat.h` entirely and their corresponding tests.
Differential Revision: https://reviews.llvm.org/D131769
Move some types in STLExtras.h which are named and behave identically to
STL types from future standards into a dedicated header. This keeps them
organized (they are not "extras" in the same sense as most types in
STLExtras.h are) and fixes circular dependencies in future patches.
Reviewed By: dblaikie
Differential Revision: https://reviews.llvm.org/D100668