Revert "Clean up SmallString a bit"

This reverts commit r203374.

Ambiguities in assign... oh well. I'm just going to revert this and
probably not try to recommit it as it's not terribly important.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203375 91177308-0d34-0410-b5e6-96231b3b80d8