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).

llvm-svn: 79450