c++: remember candidates that we ignored

During overload resolution, we sometimes outright ignore a function in
the overload set and leave no trace of it in the candidates list, for
example when we find a perfect non-template candidate we discard all
function templates, or when the callee is a template-id we discard all
non-template functions.  We should still however make note of these
non-viable functions when diagnosing overload resolution failure, but
that's not possible if they're not present in the returned candidates
list.

To that end, this patch reworks add_candidates to add such ignored
functions to the list.  The new rr_ignored rejection reason is somewhat
of a catch-all; we could perhaps split it up into more specific rejection
reasons, but I leave that as future work.

gcc/cp/ChangeLog:

	* call.cc (enum rejection_reason_code): Add rr_ignored.
	(add_ignored_candidate): Define.
	(ignored_candidate_p): Define.
	(add_template_candidate_real): Do add_ignored_candidate
	instead of returning NULL.
	(splice_viable): Put ignored (non-viable) candidates last.
	(print_z_candidate): Handle ignored candidates.
	(build_new_function_call): Refine shortcut that calls
	cp_build_function_call_vec now that non-templates can
	appear in the candidate list for a template-id call.
	(add_candidates): Replace 'bad_fns' overload with 'bad_cands'
	candidate list.  When not considering a candidate, add it
	to the list as an ignored candidate.  Add all 'bad_cands'
	to the overload set as well.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/param-type-mismatch-2.C: Rename template
	function test_7 that (maybe accidentally) shares the same name
	as its non-template callee.
	* g++.dg/overload/error6.C: New test.

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