swift234

Several conflated changes:
 - Enhance subscript diagnostics to chase into the constraint system to find
   overload candidates that failed to match so we can rank and diagnose 
   subscript ambiguities using the same mechanics we have for operators.
 - Implement a copy of suggestPotentialOverloads based on the new mechanics
   for overload set resolution.  This allows us to diagnose these in a more
   detailed way, but for now we're keeping it as similar to the old system as
   possible.  The old version to be removed once the last client moves off it.
 - Add a bunch of testcases to decl/subscript/subscripting.swift where we are
   doing unfortunate things still.



Swift SVN r29810