[OpenMP][MLIR][Flang][bbc][Driver] Add OpenMP RTL Flags to Flang and generate...

[OpenMP][MLIR][Flang][bbc][Driver] Add OpenMP RTL Flags to Flang and generate omp.FlagsAttr from them

This patch ports OpenMP RTL flags from the shared Clang compiler
options to Flang. As well as adding a limited subset to bbc.

This patch enables the flags below (and any equals or inverse variants)
for Flang that exist in Clang:

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

For the bbc tool it only utilises the primary variants to minimize
additional complexity in the tool.

The patch also provides FlagAttr generation from these flags. Which
will be lowered to LLVM-IR in a subsequent patch.

Reviewers: kiranchandramohan, awarzynski

Differential Revision: https://reviews.llvm.org/D147324