Treat the semantics of pointee.size as "dereferenceable-at-point"
and always specify the size. Instead, use a separate NoFree attribute
to determine whether dereferenceability extends to the whole function.

Then in the LLVM backend, only actually emit dereferenceable if
nofree is also set, as dereferenceable currently implies nofree.

In addition, explicitly emit the nofree attribute, which will help
when LLVM switches dereferenceable to be at-point.