swift234

Sema: Don't favor extension methods over protocol requirements

This bug triggered when three pre-conditions held:

- A protocol requirement method signature involves archetypes built
  from associated types
- A protocol extension provides a default implementation of this method
- The method is called from another method in the same extension,
  with a parameter of the associated type

In this case, the archetypes in the signature would match exactly
and we would end up picking the extension method over the protocol
method, which would not even be considered. As a result, the
extension method would be dispatch statically instead of using
witness method dispatch, which is wrong.

Hopefully one day we can model default implementations as real
overrides and not overloads.

Note that the test is a SILGen test even though the bug is in Sema,
since its easier to detect the problem at the SIL level.

Fixes <rdar://problem/21995666>.

Swift SVN r30847