Turn globals into places?

[RalfJung]

Currently, a function-local variable is a place expression, but referencing a global works via Constant::GlobalPointer. This seems somewhat inconsistent. On the other hand, we also have Constant::FnPointer, which is nicely consistent with global data pointers (we might even want to merge data and function globals at some point). If we make globals into a place expression, we lose that symmetry between data and function globals.