Naming flag for specifying the output file name for Binary Module Interface files

[David Blaikie]

Over in https://reviews.llvm.org/D137059 we're discussing the naming
of a clang flag - would be good to have it be consistent with GCC.

The functionality is to name the BMI (.pcm in Clang's parlance) output
file when compiling a C++20 module.

Current proposal is to use `-fsave-std-cxx-module-file=` which is
certainly precise, but maybe a bit verbose. Clang has some other flags
related to modules that skip the std/cxx parts, and are just
`-fmodule-*` or `-fmodules-*`, so there's some precedent for that too.

Do GCC folks have any veto votes (is the currently proposed name
especially objectionable/wouldn't be acceptable in GCC) or preferences
(suggestions to add to the pool)?