Fix uint -> size_t conversion warning.

This warning is disabled for the LLVM build,
but external users of the header can still
run into this.

Patch by Ke Bai

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206629 91177308-0d34-0410-b5e6-96231b3b80d8