[ASTTypeTraits][ASTMatchers][OpenMP] OMPClause handling

Summary:
`OMPClause` is the base class, it is not descendant from **any**
other class, therefore for it to work with e.g.
`VariadicDynCastAllOfMatcher<>`, it needs to be handled here.

Reviewers: sbenza, bkramer, pcc, klimek, hokein, gribozavr, aaron.ballman, george.karpenkov

Reviewed By: gribozavr, aaron.ballman

Subscribers: guansong, jdoerfert, alexfh, ABataev, cfe-commits

Tags: #openmp, #clang

Differential Revision: https://reviews.llvm.org/D57112

llvm-svn: 356675