This testcase was failing because CONSTRUCTOR_IS_DESIGNATED_INIT wasn't
getting set on the introduced CONSTRUCTOR for the anonymous union, and
build_aggr_conv uses that flag to decide whether to pay attention to the
indexes of the CONSTRUCTOR. So set the flag when we see a designator rather
than relying on copying it from another CONSTRUCTOR.
PR c++/105925
gcc/cp/ChangeLog:
* decl.c (reshape_init_array_1): Set
CONSTRUCTOR_IS_DESIGNATED_INIT here.
(reshape_init_class): And here.