Fix ICE in ptr_metadata_ty_or_tail for recursive associated types

[RaghavenderSingh]

Fixes #153431.

When a struct's tail field has a recursive associated type, normalization
overflows and normalize_projection_term returns a fresh inference variable
(TyVar) instead of the original alias. This TyVar escapes struct_tail_raw
and reaches ptr_metadata_ty_or_tail, which had no arm for it and called
bug!.

Fix by handling ty::Infer(ty::TyVar(_)) the same way as ty::Error(_),
return Ok(tcx.types.unit) since the overflow error has already been reported.

[rustbot]

r? @adwinwhite

rustbot has assigned @adwinwhite.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 69 candidates
• Random selection from 12 candidates