[ASTMatchers][OpenMP] OpenMP Structured-block-related matchers

Summary: Exposes to the  for ASTMatchers the interface/modelling
of OpenMP structured-block.

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

Reviewed By: gribozavr, aaron.ballman

Subscribers: guansong, jdoerfert, cfe-commits

Tags: #clang, #openmp

Differential Revision: https://reviews.llvm.org/D59463

llvm-svn: 356676