[Clang][OpenMP] Add support for compare capture in parser

This patch adds the support for `atomic compare capture` in parser and part of
sema. We don't create an AST node for this because the spec doesn't say `compare`
and `capture` clauses should be used tightly, so we cannot look one more token
ahead in the parser.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D116261