[ADT] Add STLForwardCompat.h and llvm::disjunction

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