[OPENMP]Use standard parsing for 'match' clause, NFC.

Reused standard clauses parsing scheme for parsing/matching 'match'
clause in 'declare variant' directive.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@372635 91177308-0d34-0410-b5e6-96231b3b80d8