c++modules: report module mapper files as a dependency

c++modules: report module mapper files as a dependency

It affects the build, and if used as a static file, can reliably be
tracked using the `-MF` mechanism.

gcc/cp/:

	* mapper-client.cc, mapper-client.h (open_module_client): Accept
	dependency tracking and track module mapper files as
	dependencies.
	* module.cc (make_mapper, get_mapper): Pass the dependency
	tracking class down.

gcc/testsuite/:

	* g++.dg/modules/depreport-2.modmap: New test.
	* g++.dg/modules/depreport-2_a.C: New test.
	* g++.dg/modules/depreport-2_b.C: New test.
	* g++.dg/modules/test-depfile.py: Support `:|` syntax output
	when generating modules.

Signed-off-by: Ben Boeckel <ben.boeckel@kitware.com>
Reviewed-by: Jason Merrill <jason@redhat.com>