[OpenMP][MLIR] Add lowering of omp::FlagsAttr to LLVM-IR

The omp::FlagsAttr contains OpenMP RTL flags
given by a user to the compiler and a frontend
(flang currently) then populates the omp::FlagsAttr
which will then lower these to LLVM globals
which are utilised by the OpenMP runtime.

Reviewers: jdoerfert

Differential Revision: https://reviews.llvm.org/D147344