Fix type truncation warnings

Avoid type truncation warnings from a 32-bit bot due to size_t not
being unsigned long long, by converting the variables and constants to
unsigned. This was introduced by r278338 and caused warnings here:
http://bb.pgr.jp/builders/i686-mingw32-RA-on-linux/builds/15527/steps/build_llvmclang/logs/warnings%20%287%29

llvm-svn: 278406