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

stdlib/objc/Foundation/NSStringAPI.swift

@@ -995,7 +995,7 @@ extension String {
   /// Returns an array of NSString objects containing, in
   /// order, each path component of the `String`.
   var pathComponents: String[] {
-    return (_ns.pathComponents as String[])!
+    return _ns.pathComponents as String[]
   }
 
   // @property NSString* pathExtension;
@@ -1353,7 +1353,7 @@ extension String {
   /// Returns an array of strings made by separately appending
   /// to the `String` each string in in a given array.
   func stringsByAppendingPaths(paths: String[]) -> String[] {
-    return (_ns.stringsByAppendingPaths(paths) as String[])!
+    return _ns.stringsByAppendingPaths(paths) as String[]
   }
 
   // - (NSString *)substringFromIndex:(NSUInteger)anIndex