c++: designated init and aggregate members [PR103337]

Our C++20 designated initializer handling was broken with members of class
type; we would find the relevant member and then try to find a member of
the member with the same name.  Or we would sometimes ignore the designator
entirely.  The former problem is fixed by the change to reshape_init_class,
the latter by the change to reshape_init_r.

	PR c++/103337
	PR c++/102740
	PR c++/103299
	PR c++/102538

gcc/cp/ChangeLog:

	* decl.cc (reshape_init_class): Avoid looking for designator
	after we found it.
	(reshape_init_r): Keep looking for designator.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/flexary3.C: Remove one error.
	* g++.dg/parse/pr43765.C: Likewise.
	* g++.dg/cpp2a/desig22.C: New test.
	* g++.dg/cpp2a/desig23.C: New test.
	* g++.dg/cpp2a/desig24.C: New test.
	* g++.dg/cpp2a/desig25.C: New test.