[libomptarget][nfc] Make interface.h target independent

Summary:
[libomptarget][nfc] Make interface.h target independent

Move interface.h under a top level include directory.
Remove #includes to avoid the interface depending on the implementation.

Reviewers: ABataev, jdoerfert, grokos, ronlieb, RaviNarayanaswamy

Reviewed By: jdoerfert

Subscribers: mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D68615

llvm-svn: 374919