c++: SFINAE for invalid delete-expression [PR79706]

c++: SFINAE for invalid delete-expression [PR79706]

This fixes SFINAE when substitution yields an invalid delete-expression
due to the pertinent deallocation function being marked deleted or
otherwise inaccessible.

We need to check for an erroneous result from build_op_delete_call and
exit early in that case, so that we don't build a COND_EXPR around the
erroneous result which finish_decltype_type would then quietly accept.

gcc/cp/ChangeLog:

	PR c++/79706
	* init.c (build_vec_delete_1): Just return error_mark_node if
	deallocate_expr is error_mark_node.
	(build_delete): Just return error_mark_node if do_delete is
	error_mark_node.

gcc/testsuite/ChangeLog:

	PR c++/79706
	* g++.dg/template/sfinae30.C: New test.