[OpenMP][MLIR] Add Flags attribute to OMP OffloadModuleInterface

The intent of this attribute is for it to be applied to a module and
then hold information on runtime library (RTL) flags given to
Flang (or other OpenMP frontend) that should be lowered down to
LLVM-IR for devices as LLVM globals. The following related
flags are:

-fopenmp-target-debug
-fopenmp-assume-threads-oversubscription
-fopenmp-assume-teams-oversubscription
-fopenmp-assume-no-nested-parallelism
-fopenmp-assume-no-thread-state

These exist within Clang and are lowered into the IR when
offloading for device. This attribute allows this infromation
to be carried down from the Flang frontend to the
LLVM/OpenMP Dialect to LLVM-IR translation phase
and then be lowered to LLVM-IR.

Reviewers: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D144896