Fix layering StringRef copy using BumpPtrAllocator.

Now to copy a string into a BumpPtrAllocator and get a StringRef to the copy:

   StringRef myCopy = myStr.copy(myAllocator);
   



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200885 91177308-0d34-0410-b5e6-96231b3b80d8