c++: print z candidate count and number them (v2)

c++: print z candidate count and number them (v2)

Changed in v2: changed wording to "there is"/"there are" rather
than "we found".

This patch is a followup to:
  "c++: use diagnostic nesting [PR116253]"

Following Sy Brand's UX suggestions in P2429R0 for example 1, this patch
tweaks print_z_candidates to add a note about the number of candidates,
and adds a candidate number to each one.

Various examples of output can be seen in the testsuite part of the
patch.

gcc/cp/ChangeLog:
	* call.cc (print_z_candidates): Count the number of
	candidates and issue a note stating the count at an
	intermediate nesting level.  Number the individual
	candidates.

gcc/testsuite/ChangeLog:
	* g++.dg/concepts/diagnostic9.C: Update expected
	results for candidate count and numbering.
	* g++.dg/concepts/nested-diagnostics-1-truncated.C:
	* g++.dg/concepts/nested-diagnostics-1.C: Likewise.
	* g++.dg/concepts/nested-diagnostics-2.C: Likewise.
	* g++.dg/cpp23/explicit-obj-lambda11.C: Likewise.
	* g++.dg/cpp2a/desig4.C: Likewise.
	* g++.dg/cpp2a/desig6.C: Likewise.
	* g++.dg/cpp2a/spaceship-eq15.C: Likewise.
	* g++.dg/diagnostic/function-color1.C: Likewise.
	* g++.dg/diagnostic/param-type-mismatch-2.C: Likewise.
	* g++.dg/diagnostic/pr100716-1.C: Likewise.
	* g++.dg/diagnostic/pr100716.C: Likewise.
	* g++.dg/lookup/operator-2.C: Likewise.
	* g++.dg/lookup/pr80891-5.C: Likewise.
	* g++.dg/modules/adhoc-1_b.C: Likewise.
	* g++.dg/modules/err-1_c.C: Likewise.
	* g++.dg/modules/err-1_d.C: Likewise.
	* g++.dg/other/return2.C: Likewise.
	* g++.dg/overload/error6.C: Likewise.
	* g++.dg/template/local6.C: Likewise.

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