Change the meaning of "if let x = foo()" back to Xcode 6.4 semantics. The compiler

includes a number of QoI things to help people write the correct code.  I will commit
the testcase for it as the next patch.

The bulk of this patch is moving the stdlib, testsuite and validation testsuite to
the new syntax.  I moved a few uses of "as" patterns back to as? expressions in the 
stdlib as well.



Swift SVN r27959

validation-test/compiler_crashers/0196-swift-constraints-constraint-create.swift

@@ -275,7 +275,7 @@ class b: a {
 b
 protocol c : b { func b
 func some<S: SequenceType, T where Optional<T> == S.Generator.Element>(xs : S) -> T? {
-    for (mx :       if let x? = mx {
+    for (mx :       if let x = mx {
      d: f{  ceanTy b {
     clasi() {
     }