[OpenMP] Introduce the `disable_selector_propagation` variant selector trait

Nested `omp [begin|end] declare variant` inherit the selectors from
surrounding `omp (begin|end) declare variant` constructs. To stop such
propagation the user can add the `disable_selector_propagation` to the
`extension` set in the `implementation` selector.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D95765