This patch enables the `-fopenmp-force-usm` option to be passed to the
flang driver, which forwards it to the compiler frontend. This flag,
when set, results in the introduction of the `unified_shared_memory` bit
to the `omp.requires` attribute of the top-level module operation.
This is later combined with any other target device-related REQUIRES
clauses that may have been explicitly set in the compilation unit.
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