Rollup merge of #115631 - compiler-errors:ctypes-unsized, r=davidtwco

Don't ICE when computing ctype's `repr_nullable_ptr` for possibly-unsized ty

We may not always be able to compute the layout of a type like `&T` when `T: ?Sized`, even if we're able to estimate its size skeleton.

r? davidtwco

Fixes #115628

Author: matthiaskrgr