This adds initial support for letting targets specify which address
spaces their functions should reside in by default.
If a function is created by a frontend, it will get the default address space specified in the DataLayout, unless the frontend explicitly uses a more general llvm::Function constructor. Function address spaces will become a part of the bitcode and textual IR forms, as we do not have access to a data layout whilst parsing LL.
It will be possible to write IR that explicitly has addrspace(n) on a function. In this case, the function will reside in the specified space, ignoring the default in the DL.
This is the first step towards placing functions into the correct
address space for Harvard architectures.