`DIExpression` deals with `uint64_t`, so it doesn't make sense that
`createExpression()` is created from `int64_t`. Switch to `uint64_t` to
unify them.
I've temporarily left in the `int64_t` version, which forwards to the
`uint64_t` version. I'll delete it once I've updated the callers.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228619 91177308-0d34-0410-b5e6-96231b3b80d8