[OpenMP] "UnFix" layering problem with FrontendOpenMP

This reverts commit 97aa593a as it
causes problems (PR45453) https://reviews.llvm.org/D77574#1966321.

This additionally adds an explicit reference to FrontendOpenMP to
clang-tidy where ASTMatchers is used.

This is hopefully just a temporary solution. The dependence on
`FrontendOpenMP` from `ASTMatchers` should be handled by CMake
implicitly, not us explicitly.

Reviewed By: aheejin

Differential Revision: https://reviews.llvm.org/D77666