The `-fwritable-strings' flag doesn't make the backing store strings of all

CFStrings writable.

The strings (both Unicode and ASCII) should reside in a read-only section. E.g.,
__TEXT,__cstring instead of __DATA,__data. This is done by making the global
variable created for the strings constant despite the value of that flag.
<rdar://problem/10657500>

llvm-svn: 147845