Implement the new casting syntaxes "as" and "as?".

There's a bit of a reshuffle of the ExplicitCastExpr subclasses:
  - The existing ConditionalCheckedCastExpr expression node now represents
"as?". 
  - A new ForcedCheckedCastExpr node represents "as" when it is a
  downcast.
  - CoerceExpr represents "as" when it is a coercion.
  - A new UnresolvedCheckedCastExpr node describes "as" before it has
  been type-checked down to ForcedCheckedCastExpr or CoerceExpr. This
  wasn't a strictly necessary change, but it helps us detangle what's
  going on.

There are a few new diagnostics to help users avoid getting bitten by
as/as? mistakes:
  - Custom errors when a forced downcast (as) is used as the operand
  of postfix '!' or '?', with Fix-Its to remove the '!' or make the
  downcast conditional (with as?), respectively.
  - A warning when a forced downcast is injected into an optional,
  with a suggestion to use a conditional downcast.
  - A new error when the postfix '!' is used for a contextual
  downcast, with a Fix-It to replace it with "as T" with the
  contextual type T.

Lots of test updates, none of which felt like regressions. The new
tests are in test/expr/cast/optionals.swift. 

Addresses <rdar://problem/17000058>


Swift SVN r18556

test/stdlib/NSObject.swift

@@ -162,7 +162,7 @@ class ValueLike : NSObject {
   }
 
   override func isEqual(_ rhs: AnyObject) -> Bool {
-    if let rhs2 = rhs as ValueLike {
+    if let rhs2 = rhs as? ValueLike {
       return x == rhs2.x
     }
     return false