diagnostics: Make line-ending logic consistent with libcpp [PR91733]

diagnostics: Make line-ending logic consistent with libcpp [PR91733]

libcpp recognizes a lone \r as a valid line ending, so the infrastructure
for retrieving source lines to be output in diagnostics needs to do the
same. This patch fixes file_cache_slot::get_next_line() accordingly so that
diagnostics display the correct part of the source when \r line endings are in
use.

gcc/ChangeLog:

	PR preprocessor/91733
	* input.cc (find_end_of_line): New helper function.
	(file_cache_slot::get_next_line): Recognize \r as a line ending.
	* diagnostic-show-locus.cc (test_escaping_bytes_1): Adapt selftest
	since \r will now be interpreted as a line-ending.

gcc/testsuite/ChangeLog:

	PR preprocessor/91733
	* c-c++-common/pr91733.c: New test.