There is a trend in the broader C++ community to use a different extension
for module interface units, even though (in GCC) they are compiled in the
same way as other source files.  Let's recognize these extensions as C++.

.ixx is the MSVC standard, while the .c*m are supported by Clang.  libc++
standard headers use .cppm, as their other source files use .cpp.
Perhaps libstdc++ might use .ccm for parallel consistency?

One issue with .c++m is that libcpp/mkdeps.cc has been using it for the
phony dependencies to express module dependencies, so I'm changing mkdeps to
something less likely to be an actual file, ".c++-module".

gcc/cp/ChangeLog:

	* lang-specs.h: Add module interface extensions.

gcc/ChangeLog:

	* doc/invoke.texi: Update module extension docs.

libcpp/ChangeLog:

	* mkdeps.cc (make_write): Change .c++m to .c++-module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dep-1_a.C
	* g++.dg/modules/dep-1_b.C
	* g++.dg/modules/dep-2.C: Change .c++m to .c++-module.