[preprocessor] Include stacking

https://gcc.gnu.org/ml/gcc-patches/2019-08/msg01971.html
	* internal.h (enum include_type): Add IT_MAIN, IT_DIRECTIVE_HWM,
	IT_HEADER_HWM.
	(_cpp_stack_file): Take include_type, not a bool.
	* files.c (_cpp_find_file): Refactor to not hide an if inside a
	for conditional.
	(should_stack_file): Break apart to ...
	(is_known_idempotent_file, has_unique_contents): ... these.
	(_cpp_stack_file): Replace IMPORT boolean with include_type enum.
	Refactor to use new predicates.  Do linemap compensation here ...
	(_cpp_stack_include): ... not here.
	* init.c (cpp_read_main_file): Pass IT_MAIN to _cpp_stack_file.

From-SVN: r275034