Prevent fix-it hints from affecting more than one line

Prevent fix-it hints from affecting more than one line

Attempts to apply a removal or replacement fix-it hint to a source
range that covers multiple lines currently lead to nonsensical
results from the printing code in diagnostic-show-locus.c.

We were already filtering them out in edit-context.c (leading
to -fdiagnostics-generate-patch not generating any output for
the whole TU).

Reject attempts to add such fix-it hints within rich_location,
fixing the diagnostic-show-locus.c issue.

gcc/ChangeLog:
	* diagnostic-show-locus.c
	(selftest::test_fixit_deletion_affecting_newline): New function.
	(selftest::diagnostic_show_locus_c_tests): Call it.

libcpp/ChangeLog:
	* include/line-map.h (class rich_location): Document that attempts
	to delete or replace a range *affecting* multiple lines will fail.
	* line-map.c (rich_location::maybe_add_fixit): Implement this
	restriction.

From-SVN: r249403