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_fixed/0194-swift-parser-parseexprsequence.swift

@@ -25,7 +25,7 @@ protocolias h
 }
 func some<S: SequenceType, T where Optional<T> == S.Generator.Element>(xs : S) -> T? {
     for (mx : T?) in xs {
-        if let x? = mx {
+        if let x = mx {
             return x
         }
     }