Fixup r303240: Use llvm::to_string instead of std::to_string

It turns out some of the buildbots don't have std::to_string around,
even in this day and age...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303243 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed