StringSaver now always saves to a BumpPtrAllocator.
The only reason for having the virtual saveImpl is so lld can have a
thread safe version.
The reason for the distinct BumpPtrStringSaver class is to avoid the
virtual destructor.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239669 91177308-0d34-0410-b5e6-96231b3b80d8
