c++: -include and header unit translation

c++: -include and header unit translation

 Within a source file, #include is translated to import if a suitable header
 unit is available, but this wasn't working with -include.  This turned out
 to be because we suppressed the translation before the beginning of the
 main file.  After removing that, I had to tweak libcpp file handling to
 accommodate the way it moves from an -include to the main file.

gcc/ChangeLog:

	* doc/invoke.texi (C++ Modules): Mention -include.

gcc/cp/ChangeLog:

	* module.cc (maybe_translate_include): Allow before the main file.

libcpp/ChangeLog:

	* files.cc (_cpp_stack_file): LC_ENTER for -include header unit.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dashinclude-1_b.C: New test.
	* g++.dg/modules/dashinclude-1_a.H: New test.