[OPENMP50]Add device/kind context selector support.

Summary: Added basic parsing/sema support for device/kind context selector.

Reviewers: jdoerfert

Subscribers: rampitec, aheejin, fedor.sergeev, simoncook, guansong, s.egerton, hfinkel, kkwli0, caomhin, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70245