Use `TypeVisitor::BreakTy` in `ProhibitOpaqueVisitor`
Use `TypeVisitor::BreakTy` in `ProhibitOpaqueVisitor`

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust