c++: Fix up cp_lexer_safe_previous_token [PR96328]

c++: Fix up cp_lexer_safe_previous_token [PR96328]

The following testcase ICEs, because cp_lexer_safe_previous_token calls
cp_lexer_previous_token and that ICEs, because all tokens in the lexer
buffer before the current one (CPP_EOF) have been purged.

cp_lexer_safe_previous_token is used in the context where it is ok if it
punts, so the patch changes the function so that it doesn't assert there is
some previous token, but instead returns NULL like in other cases where it
punts.

In addition to this, in the last hunk it does a micro-optimization, don't
call the potentially expensive function if it will not need the result,
instead check the least expensive condition first.

And the middle hunk is a similar change from Mark's version of the patch,
to use the safe variant in there because it is again just about a hint
and it is better not to provide the hint than to ICE, though we don't have a
testcase that would ICE.

2020-07-28  Jakub Jelinek  <jakub@redhat.com>
	    Mark Wielaard  <mark@klomp.org>

	PR c++/96328
	* parser.c (cp_lexer_safe_previous_token): Don't call
	cp_lexer_previous_token, instead inline it by hand and return NULL
	instead of failing assertion if all previous tokens until the first
	one are purged.
	(cp_parser_error_1): Optimize - only call cp_lexer_safe_previous_token
	if token->type is CPP_NAME.  Use cp_lexer_safe_previous_token instead
	of cp_lexer_previous_token for the missing_token_desc != RT_NONE
	case too.

	* g++.dg/diagnostic/pr96328.C: New test.

Co-Authored-By: Mark Wielaard <mark@klomp.org>