libcpp: Fix up raw string literal parsing error-recovery [PR96323]

libcpp: Fix up raw string literal parsing error-recovery [PR96323]

For (invalid) newline inside of the raw string literal delimiter, doing
continue means we skip the needed processing of newlines.  Instead of
duplicating that, this patch just doesn't continue for those.

2020-07-28  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/96323
	* lex.c (lex_raw_string): For c == '\n' don't continue after reporting
	an prefix delimiter error.

	* c-c++-common/cpp/pr96323.c: New test.