Fix many -Wsign-compare and -Wtautological-constant-compare warnings.

Fix many -Wsign-compare and -Wtautological-constant-compare warnings.

Most of the -Wsign-compare warnings are due to the fact that
enums are signed by default in the MS ABI, while the
tautological comparison warnings trigger on x86 builds where
sizeof(size_t) is 4 bytes, so N > numeric_limits<unsigned>::max()
is always false.

Differential Revision: https://reviews.llvm.org/D41256

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320750 91177308-0d34-0410-b5e6-96231b3b80d8