v0 mangling support for const generics

[ElectronicRU]

As we're currently using a const generic type parametrized by enums for Image types, v0 mangling cannot be fully supported because it doesn't handle non-basic-type const generic parameters and panics.

Once rust-lang/rust#61486 is implemented, we can switch back to v0 support.

[eddyb]

Self-assigned because I've been working on this (see rust-lang/rust#87194).