Fix some -Wstring-conversion warnings

I don't seem to see these locally, maybe just need to update my
compiler, or we haven't turned them on for LLVM's build and we should...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259146 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed