Don't allow impls to force public types

This code in resolve accidentally forced all types with an impl to become
public. This fixes it by default inheriting the privacy of what was previously
there and then becoming `true` if nothing else exits.

Closes #10545
auto merge of #11019 : alexcrichton/rust/issue-10545, r=pcwalton

This code in resolve accidentally forced all types with an impl to become
public. This fixes it by default inheriting the privacy of what was previously
there and then becoming `true` if nothing else exits.

Closes #10545