fix-it hints can't contain newlines

fix-it hints can't contain newlines

I hope to implement newline support within fix-it hints at some point,
but currently it's not supported, and leads to misleading diagnostic
output, so for now, fail gracefully.

gcc/ChangeLog:
	* diagnostic-show-locus.c
	(selftest::test_fixit_insert_containing_newline): New function.
	(selftest::test_fixit_replace_containing_newline): New function.
	(selftest::diagnostic_show_locus_c_tests): Call the above.

libcpp/ChangeLog:
	* include/line-map.h (class rich_location): Note that newlines
	aren't supported in fix-it text.
	* line-map.c (rich_location::add_fixit_insert_before): Reject
	attempts to add fix-its containing newlines.
	(rich_location::add_fixit_replace): Likewise.

From-SVN: r240169