Add Target hooks for IRgen of [cf]string literals.

Add Target hooks for IRgen of [cf]string literals.
 - Notably, set section on cfstring literal string data (for now, this
   is done everywhere because it matches what we were already doing
   for the CFString data itself)

 - <rdar://problem/6599098> [irgen] linker requires objc string data
   to go into cstring

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@68160 91177308-0d34-0410-b5e6-96231b3b80d8