swift234

Provide fix-its when implementing a protocol method with bridged types. (#2799)

This is the protocol version of 7bfdd4a2: if a protocol requirement
has a newly-bridged type (say, 'URL' instead of 'NSURL'), then any
conforming types will need to update their implementations of the
requirement. Reuse the override-checking mechanism to do so when
we're reasonably confident about it.

This slots the checking into the existing protocol diagnostics, which
doesn't result in the best user experience.

    note: protocol requires property 'prop' with type 'Refrigerator?'
      var prop: Refrigerator? { get }
          ^
    note: candidate has non-matching type 'APPRefrigerator?'
      var prop: APPRefrigerator? {
          ^     ~~~~~~~~~~~~~~~~
                Refrigerator?

But we can come back and improve that later; right now this is better
than nothing.

rdar://problem/26237030