swift234

Make conversion diagnostics between as? and as! expression consistent by allowing
diagnoseGeneralConversionFailure() to handle them (instead of it handling as? but
special code handling as!).

As part of this, enhance things so we get error messages about both the problem, 
and the overall type involved (when they're different) e.g.:

  if let s = setD as? Set<BridgedToObjC> { }

error: 'ObjC' is not a subtype of 'DerivesObjC'
note: in cast from type 'Set<DerivesObjC>' to 'Set<BridgedToObjC>'

This also finally fixes the case in test/Generics/existential_restrictions.swift



Swift SVN r31299