IR: Take uint64_t in DIBuilder::createExpression()

`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.

llvm-svn: 228619