OpenMP 5.0 metadirective

This patch supports OpenMP 5.0 metadirective features.
It is implemented keeping the OpenMP 5.1 features like dynamic user condition in mind.

A new function, getBestWhenMatchForContext, is defined in llvm/Frontend/OpenMP/OMPContext.h

Currently this function return the index of the when clause with the highest score from the ones applicable in the Context.
But this function is declared with an array which can be used in OpenMP 5.1 implementation to select all the valid when clauses which can be resolved in runtime. Currently this array is set to null by default and its implementation is left for future.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D91944