SI-7120 Erasure must honor typeref prefixes

Erasure was discarding these, which led to unnecessarily
wide types in quite particular circumstances. This showed
up as a double definition error in the reported bug when the
bridge method clashed with the erased signature.

Targeted at 2.11 rather than 2.10.2 on grounds of binary compatibility.

Review by @odersky, @paulp (should we use `dealiasWiden` here?)