Now that enough yaks are cleanly shaven, completely reimplement how

we process contextual constraints when producing diagnostic.  Formerly,
we would aggressively drop contextual type information on the floor under
the idea that it would reduce constraints on the system and make it more
likely to be solvable.  However, this also has the downside of introducing
ambiguity into the system, and some expr nodes (notably closures) cannot
usually be solved without that contextual information.

In the new model, expr diagnostics are expected to handle the fact that
contextual information may be present, and bail out without diagnosing an
error if that is the case.  This gets us more information into closures,
allowing more specific return type information, e.g. in the case in
test/expr/closure/closures.swift.

This approach also produces more correct diagnostics in a bunch of other
cases as well, e.g.:

-  var c = [:]  // expected-error {{type '[_ : _]' does not conform to protocol 'DictionaryLiteralConvertible'}} 
+  var c = [:]  // expected-error {{expression type '[_ : _]' is ambiguous without more context}}

and the examples in test/stmt/foreach.swift, test/expr/cast/as_coerce.swift,
test/expr/cast/array_iteration.swift, etc.

That said, this another two steps forward, one back thing.  Because we
don't handle propagating sametype constraints from results of calls to their
arguments, we regress a couple of (admittedly weird) cases.  This is now 
tracked by:
<rdar://problem/22333090> QoI: Propagate contextual information in a call to operands

There is also the one-off narrow case tracked by:
<rdar://problem/22333281> QoI: improve diagnostic when contextual type of closure disagrees with arguments



Swift SVN r31319

test/expr/closure/basic.swift

@@ -26,7 +26,7 @@ func variadic() {
   f(1, 2)
   f(1, 3)
 
-  let D = { (Ss ...) in 1 } // expected-error{{'...' cannot be applied to a subpattern which is not explicitly typed}}, expected-error{{unable to infer closure type in the current context}}
+  let D = { (Ss ...) in 1 } // expected-error{{'...' cannot be applied to a subpattern which is not explicitly typed}}, expected-error{{unable to infer closure return type in current context}}
 }
 
 // Closures with attributes in the parameter list.