[OpenMP][Flang][MLIR] Lowering of OpenMP requires directive from parse tree to MLIR

This patch implements the lowering of the OpenMP 'requires' directive
from Flang parse tree to MLIR attributes attached to the top-level
module.

Target-related 'requires' clauses are gathered and combined for each top-level
unit during semantics. Lastly, a single module-level `omp.requires` attribute
is attached to the MLIR module with that information at the end of the process.

The `atomic_default_mem_order` clause is not addressed by this patch, but
rather it will come as a separate patch and follow a different approach.

Depends on D147214, D150328, D150329 and D157983.

Differential Revision: https://reviews.llvm.org/D147218