c++: Note macro locations

In order to prune ordinary locations, we need to note the locations of
macros we'll be writing out.  This rearanges the macro processing to achieve
that.  Also drop an unneeded parameter from macro reading & writing.

Fix some it's/its errors.

	gcc/cp/
	* module.cc (module_state::write_define): Drop located param.
	(module_state::read_define): Likewise.
	(module_state::prepare_macros): New, broken out of ...
	(module_state::write_macros): ... here.  Adjust.
	(module_state::write_begin): Adjust.
	gcc/testsuite/
	* g++.dg/modules/inext-1.H: Check include-next happened.