[OpenMP] Add checks for requires and target directives.

Summary: The requires directive containing target related clauses must appear before any target region in the compilation unit.

Reviewers: ABataev, AlexEichenberger, caomhin

Reviewed By: ABataev

Subscribers: guansong, jfb, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60875

llvm-svn: 358709