c++: anon union designated init [PR105925]

c++: anon union designated init [PR105925]

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.

This avoids some redundant errors on desig4.C because we stop setting
CONSTRUCTOR_IS_DESIGNATED_INIT on _Complex CONSTRUCTORs where it's
nonsense.

	PR c++/105925

gcc/cp/ChangeLog:

	* decl.cc (reshape_init_array_1): Set
	CONSTRUCTOR_IS_DESIGNATED_INIT here.
	(reshape_init_class): And here.
	(reshape_init): Not here.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/desig4.C: Remove extra errors.
	* g++.dg/cpp2a/desig26.C: New test.