[OpenMP][Flang][MLIR] Implement OffloadModuleInterface for OpenMP Dialect and...

[OpenMP][Flang][MLIR] Implement OffloadModuleInterface for OpenMP Dialect and convert is_device to an Attribute

This commit adds the OffloadModuleInterface to the OpenMP dialect,
which will implement future module attribute get/set's for offloading.
Currently it implements set and get's for the omp.is_device attribute,
which is promoted to a real attribute in this commit as well (primarily
to allow switch cases to work nicely with it for future work and to keep
consistency with future module attributes).

This interface is attached to mlir::ModuleOp's on registration of the
OpenMPDialect and should be accessible anywhere the OpenMP
dialect is registered and initialized.

Reviewers: kiranchandramohan, awarzynski

Differential Revision: https://reviews.llvm.org/D146850