swift234

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