c++: desig init in presence of list ctor [PR109871]

c++: desig init in presence of list ctor [PR109871]

add_list_candidates has logic to reject designated initialization of a
non-aggregate type, but this is inadvertently being suppressed if the type
has a list constructor due to the order of case analysis, which in the
below testcase leads to us incorrectly treating the initializer list as if
it's non-designated.  This patch fixes this by making us check for invalid
designated initialization sooner.

	PR c++/109871

gcc/cp/ChangeLog:

	* call.cc (add_list_candidates): Check for invalid designated
	initialization sooner and even for types that have a list
	constructor.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/desig27.C: New test.