[flang][driver] Add support for `-mllvm`

This option is added in both `flang-new` (the compiler driver) and
`flang-new -fc1` (the frontend driver). The semantics are consistent
with `clang` and `clang -cc1`.

As Flang does not run any LLVM passes when invoked with `-emit-llvm`
(i.e. `flang-new -S -emit-llvm <file>`), the tests use
`-S`/`-c`/`-emit-obj` instead. These options require an LLVM backend to
be run by the driver to generate the output (this makese `-mllvm`
relevant here).

Differential Revision: https://reviews.llvm.org/D121374