diagnostics: fix ICE on sarif output when source file is unreadable [PR111700]

diagnostics: fix ICE on sarif output when source file is unreadable [PR111700]

gcc/ChangeLog:
	PR driver/111700
	* input.cc (file_cache::add_file): Update leading comment to
	clarify that it can fail.
	(file_cache::lookup_or_add_file): Likewise.
	(file_cache::get_source_file_content): Gracefully handle
	lookup_or_add_file failing.

gcc/testsuite/ChangeLog:
	PR driver/111700
	* c-c++-common/diagnostic-format-sarif-file-pr111700.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>