LLVM assertion when initializing a constant function pointer

```
rusti> extern fn foopy() {}
()
rusti> const f: *u8 = foopy;
rusti: /run/media/jdm/ssd/rust/src/llvm/lib/VMCore/Globals.cpp:168: void llvm::GlobalVariable::setInitializer(llvm::Constant*): Assertion `InitVal->getType() == getType()->getElementType() && "Initializer type must match GlobalVariable type"' failed.
Aborted (core dumped)
```
