c++: Distinguish ambiguity from no valid candidate

Several recent C++ features are specified to try overload resolution, and if
no viable candidate is found, do something else.  But our error return
doesn't distinguish between that situation and finding multiple viable
candidates that end up being ambiguous.  We're already trying to separately
return the single function we found even if it ends up being ill-formed for
some reason; for ambiguity let's pass back error_mark_node, to be
distinguished from NULL_TREE meaning no viable candidate.

gcc/cp/ChangeLog:

	* call.c (build_new_op_1): Set *overload for ambiguity.
	(build_new_method_call_1): Likewise.

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