Fix ICE with unsized type in const pattern
Fixes#87046. The `deref_const()` query currently contains the following check:
e9a387d6cf/compiler/rustc_mir/src/const_eval/mod.rs (L191-L204)
i.e. this will cause an ICE for every unsized type except slices. An error is reported with my changes if such a type is used as a const pattern (this should not be a breaking change, since so far, this has caused an ICE).