[OPENMP50]Generalize handling of context matching/scoring.

Summary:
Untie context matching/scoring from the attribute for declare variant
directive to simplify future uses in other context-dependent directives.

Reviewers: jdoerfert

Subscribers: guansong, kkwli0, caomhin, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69952