c++: Improve diagnostic for implicit conversion errors [PR115163]

c++: Improve diagnostic for implicit conversion errors [PR115163]

This patch adds a note to indicate if any viable explicit conversion
functions were skipped if an implicit conversion failed to occur.

Perhaps the base diagnostic in ocp_convert can be further improved for
class types as well, as the current message is not very clear, but I've
not looked into that for this patch.

	PR c++/115163

gcc/cp/ChangeLog:

	* call.cc (implicit_conversion_error): Add flags argument, call
	maybe_show_nonconverting_candidate.
	(build_converted_constant_expr_internal): Pass flags to
	implicit_conversion_error.
	(perform_implicit_conversion_flags): Likewise.
	* cvt.cc (ocp_convert): Call maybe_show_nonconverting_candidate
	on conversion error.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible7.C: Add new testcases.
	* g++.dg/diagnostic/explicit2.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>