c++: fix count of z candidates for non-viable candidates, nesting [PR121966]

c++: fix count of z candidates for non-viable candidates, nesting [PR121966]

In r15-6116-gd3dd24acd74605 I updated print_z_candidates to show the
number of candidates, and a number for each candidate.

PR c++/121966 notes that the printed count is sometimes higher than
what's actually printed: I missed the case where candidates in the
list aren't printed due to not being viable.

Fixed thusly.

gcc/cp/ChangeLog:
	PR c++/121966
	* call.cc (print_z_candidates): Copy the filtering logic on viable
	candidates from the printing loop to the counting loop, so that
	num_candidates matches the number of iterations of the latter
	loop.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>