Partially initialized datatypes are prone to crashes

[Keno]

julia> struct Foo <: AbstractVector{fieldtype(Foo, :x)}; end
ERROR:
[17369] signal 11 (2): Segmentation fault: 11
in expression starting at REPL[1]:1

julia> struct Foo
          x::(struct Bar; y::Foo; end; println(Core.sizeof(Bar)); println(Core.sizeof(Foo)); Bar)
       end
8

[745382] signal 11 (1): Segmentation fault
in expression starting at REPL[1]:1

We should look at this with #60569.