Fix the -Werror -Wpedantic clang selfhost build

This is a stopgap fix for cast warnings introduced in r192864.

A proper fix should be investigated by the author when possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193160 91177308-0d34-0410-b5e6-96231b3b80d8