Introduce class edit_context

Introduce class edit_context

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Add edit-context.o.
	* diagnostic-color.c (color_dict): Add "diff-filename",
	"diff-hunk", "diff-delete", and "diff-insert".
	(parse_gcc_colors): Update default value of GCC_COLORS in comment
	to reflect above changes.
	* doc/invoke.texi (-fdiagnostics-color): Update description of
	default GCC_COLORS, and of the supported capabilities.
	* edit-context.c: New file.
	* edit-context.h: New file.
	* input.c (struct fcache): Add field "missing_trailing_newline".
	(diagnostics_file_cache_forcibly_evict_file): Initialize it to
	true.
	(add_file_to_cache_tab): Likewise.
	(fcache::fcache): Likewise.
	(get_next_line): Update c->missing_trailing_newline.
	(location_missing_trailing_newline): New function.
	* input.h (location_missing_trailing_newline): New decl.
	* selftest-run-tests.c (selftest::run_tests): Call
	edit_context_c_tests.
	* selftest.h (edit_context_c_tests): New decl.

libcpp/ChangeLog:
	* include/line-map.h (rich_location::seen_impossible_fixit_p): New
	accessor.

From-SVN: r239963