[flang][driver] Add support for consuming LLVM IR/BC files

This change makes sure that Flang's driver recognises LLVM IR and BC as
supported file formats. To this end, `isFortran` is extended and renamed
as `isSupportedByFlang` (the latter better reflects the new
functionality).

New tests are added to verify that the target triple is correctly
overridden by the frontend driver's default value or the value specified
with `-triple`. Strictly speaking, this is not a functionality that's
new in this patch (it was added in D124664). This patch simply enables
us to write such tests and hence I'm including them here.

Differential Revision: https://reviews.llvm.org/D124667