[OpenMP51] Initial support for masked directive and filter clause

Adds basic parsing/sema/serialization support for the #pragma omp masked
directive.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D99995

GitOrigin-RevId: 1a43fd27691c6ae5c3d725ca7c6f4fd19357f4b8
26 files changed