Simplify the raw_svector_ostream tweak from r212816

The memcpy() and overlap helps didn't help much with timings, so clean up the change.

The difference at this point is that we now leave growth of the storage buffer
up to SmallVector's implementation:

 -   OS.reserve(OS.capacity() * 2);
 +   OS.reserve(OS.size() + 64);

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212837 91177308-0d34-0410-b5e6-96231b3b80d8