c++: input_location and lookahead [PR104025]

Debug information was getting confused because input_location was different
depending on whether we had looked ahead to see if the next tokens look like
a template argument list.

I tried resetting input_location in cp_lexer_rollback_tokens itself, but
that caused regressions, so let's just do it here for now.

	PR c++/104025

gcc/cp/ChangeLog:

	* parser.cc (saved_token_sentinel::rollback): Call
	cp_lexer_set_source_position.
	(~saved_token_sentinel): Call rollback.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/pr104025.C: New test.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>