[IPO][AVR] Create new Functions in the default address space specified in the data layout

This modifies the IPO pass so that it respects any explicit function
address space specified in the data layout.

In targets with nonzero program address spaces, all functions should, by
default, be placed into the default program address space.

This is required for Harvard architectures like AVR. Without this, the
functions will be marked as residing in data space, and thus not be
callable.

This has no effect to any in-tree official backends, as none use an
explicit program address space in their data layouts.

Patch by Tim Neumann.

llvm-svn: 349469