[flang][driver] Make `flang-new -fc1` accept MLIR files

This relatively small change will allow Flang's frontend driver,
`flang-new -fc1`, to consume and parse MLIR files.  Semantically (i.e.
from user's perspective) this is identical to reading LLVM IR files.

Two file extensions are associated with MLIR files: .fir and .mlir. Note
that reading MLIR files makes only sense when running one of the
code-generation actions, i.e. when using one of the following action
flags: -S, -emit-obj, -emit-llvm, -emit-llvm-bc.

The majority of tests that required `tco` to run are updated to also run
with `flang-new -fc1`. A few tests are updated to use `fir-opt` instead
of `tco` (that's the preferred choice when testing a particular MLIR
pass). basic-program.fir is not updated as that test is intended to
verify the behaviour of `tco` specifically.

Differential Revision: https://reviews.llvm.org/D126890