[Flang][OpenMP][MLIR][Driver][bbc] Add -fopenmp-is-device flag to Flang -fc1 &...

[Flang][OpenMP][MLIR][Driver][bbc] Add -fopenmp-is-device flag to Flang -fc1 & the bbc tool, and omp.is_device attribute

Adds the -fopenmp-is-device flag to bbc and Flang's -fc1 (but not flang-new) and in addition adds an omp.is_device attribute onto the module when fopenmp is passed, this is a boolean attribute that is set to false or true dependent on if fopenmp-is-device is specified alongside the fopenmp flag on the commandline when invoking flang or bbc.

Reviewers:
awarzynski
kiranchandramohan

Differential Revision: https://reviews.llvm.org/D144864