This change is mostly about making sure that Flang's driver recognises
LLVM IR and BC as supported as supported file formats. To this end,
isFortran is extended and renamed as isSupportedByFlang (the latter
better reflects the new functionality).
Some new tests are added to verify that the target triple is overridden
by the frontend driver's default or the value specified with -triple.
Strictly speaking, this is not a functionality that's added here.
However, this patch enables us to write such tests and hence I'm
including them here.