[ASTMatchers][OpenMP] Add base ompExecutableDirective() matcher.

Summary:
A simple matcher for `OMPExecutableDirective` Stmt type.
Split off from D57113.

Reviewers: gribozavr, aaron.ballman, JonasToth, george.karpenkov

Reviewed By: gribozavr, aaron.ballman

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59453

llvm-svn: 356674