Add default address space for functions to the data layout (1/3)

Summary:
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.

Full patchset
* Add program address space to data layout D37052
* Require address space to be specified when creating functions D37054
* [clang] Require address space to be specified when creating functions D37057

Reviewers: pcc, arsenm, kparzysz, hfinkel, theraven

Reviewed By: theraven

Subscribers: arichardson, simoncook, rengolin, wdng, uabelho, bjope, asb, llvm-commits

Differential Revision: https://reviews.llvm.org/D37052

llvm-svn: 325479