Handle UTF-16 and UTF-32 constant CFStrings

We have a longstanding issue where the expression parser does not handle wide CFStrings (e.g., @"凸凹") correctly, producing the useless error message

Internal error [IRForTarget]: An Objective-C constant string's string initializer is not an array
error: warning: expression result unused
error: The expression could not be prepared to run in the target

This is just a side effect of the fact that we don't handle wide string constants when converting these to CFStringCreateWithBytes. That function takes the string's encoding as an argument, so I made it work and added a testcase.

https://reviews.llvm.org/D27291
<rdar://problem/13190557>

llvm-svn: 288386