[Sema] Allow weak/unowned types when checking Hashable/Equatable conformance

[theblixguy]

This PR fixes an issue with Equatable and Hashable conformance checking, where it won't look through a ReferenceStorageType.

The following code works as expected now:

struct Foo: Equatable, Hashable {
    weak var foo: Bar?
    unowned var bar: Bar
}

class Bar {
   let bar: String

   init(bar: String) {
     self.bar = bar
   }
}

extension Bar: Equatable, Hashable {
  static func == (lhs: Bar, rhs: Bar) -> Bool {
        return lhs.bar == rhs.bar
  }

  func hash(into hasher: inout Hasher) {}
}

[theblixguy]

cc @slavapestov @jrose-apple @xedin

[slavapestov]

Please add a test case.

[theblixguy]

@slavapestov Done :)

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

Thanks!

[dlindenkreuz]

Related Swift forums thread:
https://forums.swift.org/t/struct-with-weak-member-prevents-synthesized-equatable/19778

Thanks for fixing, @theblixguy! Let's hope this makes it to 5.0.

[theblixguy]

@dlindenkreuz This won't make 5.0, but will be available in 5.1.