[C++20][Modules][Driver][HU 3/N] Handle foo.h with -fmodule-header and/or C++ invocation.

Allow an invocation like clang -fmodule-header bar.h (which will be a C++
compilation, but using a header which will be recognised as a C one).

Also  we do not want to produce:
 "treating 'c-header' input as 'c++-header' when in C++ mode"
diagnostics when the user has been specific about the intent.

Differential Revision: https://reviews.llvm.org/D121590