[C++20] [Modules] [Driver] Emit unused argument warning if we use...

[C++20] [Modules] [Driver] Emit unused argument warning if we use '-fmodule-output' with non-module input

We required the file name of an 'importable module unit' should end
with .cppm (or .ccm, .cxxm, .c++m).

But the driver can accept '-fmodule-output' for files with normal
suffixes (e.g., .cpp). This is somewhat inconsistency.

In this patch, we only claim the option `-fmodule-output` is used if
the type of the input file is modules related. Then now the compiler
will emit 'unused argument' warnings if the input file is not modules
related.