fix some cfstring related issues:

1) -fwritable-string does affect the non-utf16 version of cfstrings
   just not the utf16 ones.
2) utf16 strings should always be marked constant, as the __TEXT segment
   is readonly.
3) The name of the global doesn't matter, remove it from TargetInfo.
4) Trust the asmprinter to drop cstrings into the right section, like llvmgcc does now.

This fixes rdar://7115750

llvm-svn: 84077