Reject attempts to initialize non-aggregate types from a designated

initializer list.

This previously led to some weird behaviors where we would unwrap the
initializer list expression and then try to use the DesignatedInitExprs
as constructor arguments.

Under the C++20 language rules, it's not valid to initialize a
reference-to-aggregate from a designated initializer list, but we have
historically accepted that, as do other compilers, and we continue to
accept that with this change. I've asked WG21 whether this should be
considered a wording defect.