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.