[Clang][OpenMP] Add static version of getSingleClause<ClauseT>. NFC.

The current method getSingleClause requires an instance of OMPExecutableDirective to be called. Introduce a static version taking a list of clauses as argument instead that can be used during parsing/Sema before any OMPExecutableDirective has been created.

This is the same approach as taken for getClausesOfKind for getting more more than a single clause of a type which also has a method and static version. NFC patch extracted out of D99459 by request.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D103665