[C++20][Modules][Driver][HU 2/N] Add fmodule-header, fmodule-header=

These command-line flags are alternates to providing the -x
c++-*-header indicators that we are building a header unit.

Act on fmodule-header= for headers on the c/l:

If we have x.hh -fmodule-header, then we should treat that header
as a header unit input (equivalent to -xc++-header-unit-header x.hh).

Likewise, for fmodule-header={user,system} the source should be now
recognised as a header unit input (since this can affect the job list
that we need).

It's not practical to recognise a header without any suffix so
-fmodule-header=system foo isn't going to happen. Although
-fmodule-header=system foo.hh will work OK.  However we can make it
work if the user indicates that the item without a suffix is a valid
header. (so -fmodule-header=system -xc++-header vector)

Differential Revision: https://reviews.llvm.org/D121589