[OpenMP][Flang][MLIR] Lowering of requires directive from MLIR to LLVM IR

Default atomic ordering information is processed in the OpenMP dialect
to LLVM IR lowering stage at every spot where an operation can be
affected by it. The rest of clauses are stored globally in the
OpenMPIRBuilderConfig object before starting that lowering stage, so
that the OMPIRBuilder can conditionally modify code generation
depending on these. At the end of the process, the omp.requires
attribute is itself lowered into a global constructor that passes these
clauses as flags to the OpenMP runtime.

Depends on D147217, D147218 and D158278.

Differential Revision: https://reviews.llvm.org/D147219