libcpp: modules and -include again

libcpp: modules and -include again

I enabled include translation to header units in r15-1104-ga29f481bbcaf2b,
but it seems that patch wasn't sufficient, as any diagnostics in the main
source file would show up as coming from the header instead.

Fixed by setting buffer->file for leaving the file transition that my
previous patch made us enter.  And don't push a buffer of newlines, in this
case that messes up line numbers instead of aligning them.

libcpp/ChangeLog:

	* files.cc (_cpp_stack_file): Handle -include of header unit more
	specially.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dashinclude-1_b.C: Add an #error.
	* g++.dg/modules/dashinclude-1_a.H: Remove dg-module-do run.