swift234

[test] Fix String test failure on some platforms.

Two of the String conversion tests want to check the case where none of CF's 
fast-access functions succeed. In one of these tests CF's fast-access functions
will always succeed when CF does not support tagged pointer string objects. 
That breaks the test on platforms that don't support tagged pointer strings. 
The test now does nothing if it does not get an NSTaggedPointerString back.


Swift SVN r18666