[OPENMP50]Add parsing/sema analysis for declare variant score.

[OPENMP50]Add parsing/sema analysis for declare variant score.

Context selectors may include optional score clause in format
`score(<expr>):`, where `<expr>` must be a constant integer expression.
Added parsing/sema analysis only.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373502 91177308-0d34-0410-b5e6-96231b3b80d8