Fix a number of clang -Wsign-compare warnings that didn't have an obvious

Fix a number of clang -Wsign-compare warnings that didn't have an obvious
solution.  The only reason these don't fire with gcc-4.2 is that gcc turns off
part of -Wsign-compare in C++ on accident.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100581 91177308-0d34-0410-b5e6-96231b3b80d8