Remove SmallString::append_*int* methods; how many copies of int -> str

Remove SmallString::append_*int* methods; how many copies of int -> str
conversion code do we really need?
 - S.append_uint(N) can be replaced with 'raw_svector_ostream(S) << N' which is
   somewhat slower due to the extra set up cost, but still plenty fast
   (especially if the svector set up cost can be amortized).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79450 91177308-0d34-0410-b5e6-96231b3b80d8