[OpenMP][Flang][MLIR] Add MLIR support for OpenMP requires directive

This patch introduces an MLIR attribute to the OpenMP dialect
representing the clauses that a 'requires' directive can define.

The `OffloadModuleInterface` is also updated to provide methods to get
and set a new dialect attribute `omp.requires`, to allow storing and using this
information during the lowering stages to LLVM IR.

Differential Revision: https://reviews.llvm.org/D147214