Remove IR::Function from the runtime.

Remove IR::Function from the runtime.

This fixes potential leaks of IR::Functions, lowers the memory usage
of the functions that the VM needs (because the IR fields are not
present in the VM::Function), and makes both managed by the module
respectively the ExecutionEngine.

Change-Id: I6748ad98b062f994eae9dd14f1919aec5aa7c0b0
Reviewed-by: Lars Knoll <lars.knoll@digia.com>