[OpenMP] Fix layering problem with FrontendOpenMP

Summary:
ASTMatchers is used in various places and it now exposes the
LLVMFrontendOpenMP library to its users without them needing to depend
on it explicitly.

Reviewers: lebedev.ri

Subscribers: mgorny, yaxunl, bollu, guansong, martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77574