[flang][driver] Make `flang` ignore `-Mfree/-Mfixed`

`-Mfixed` is not supported by the new driver and hence
`flang`, the bash wrapper script, forwards it to the host compiler.
The forwarded options are used by the host compiler when compiling the
unparsed files. As the unparsed source files are always in the free
form, forwarding `-Mfixed` is problematic.

With this patch, `-Mfixed` (and `-Mfree` for consistency) will be
ignored altogether. The user will only see a warning. This is not a
particularly sound approach, but `flang` is only a temporary solution
for us and this workaround is a fair compromise.

Differential Revision: https://reviews.llvm.org/D106428