c++: GCC accepts junk before fold-expression [PR86773]

c++: GCC accepts junk before fold-expression [PR86773]

Here we accept a bogus expression before a left fold:

Recall that a fold expression looks like:

 fold-expression:
    ( cast-expression fold-operator ... )
    ( ... fold-operator cast-expression )
    ( cast-expression fold-operator ... fold-operator cast-expression )

but here we have

    ( cast-expression ... fold-operator cast-expression )

The best fix seems to just return error_mark_node when we know this code
is invalid, and let the subsequent code report that a ) was expected.

gcc/cp/ChangeLog:

	PR c++/86773
	* parser.c (cp_parser_fold_expression): Return error_mark_node
	if a left fold is preceded by an expression.

gcc/testsuite/ChangeLog:

	PR c++/86773
	* g++.dg/cpp1z/fold12.C: New test.