swift234

A bunch of conflated changes:
 - Fix TypeCheckExpr.cpp to be more careful when propagating sugar from an 
   argument to the result of the function.  We don't want to propagate parens,
   because they show up in diagnostics later.

 - Restructure FailureDiagnosis::diagnoseFailure() to strictly process the tree
   in depth first order.  Before it would only do this if contextual typing was
   unavailable, leading to unpredictable inconsistencies between diagnostics.

 - Always perform diagnoseContextualConversionError early, as part of the thing
   that calls the visitor, instead of in each visit method.  This may change in
   the future, but is a simplification for now.

 - Make the operator processing code handle the "candidate is an exact match"
   case by emitting a diagnostic indicating that the result type of the operator
   must not match expectations, instead of emitting the silly things like
   "binary operator '&' cannot be applied to two Int operands" which is obviously
   false.

These changes lead to minor improvements across the testsuite, and should make the
diagnostics more predictable for more complex real-world ones, but I haven't gone
through the radars yet.  

Major missing pieces:
 - CallExpr isn't using the same logic that the operators are.
 - When you have a near match (only one argument mismatches) we should specifically
   complain about that argument, instead of spewing an entire argument list.
 - The noescape function attr diagnostic is being emitted twice now.
 



Swift SVN r29733