PR49020: Diagnose brace elision in designated initializers in C++.

This is a corner of the differences between C99 designators and C++20
designators that we'd previously overlooked. As with other such cases,
this continues to be permitted as an extension and allowed by default,
behind the -Wc99-designators warning flag, except in cases where it
leads to a conformance difference (such as in overload resolution and in
a SFINAE context).