[AMDGPU] Temporarily change constant address space from 4 to 2

Our final address space mapping is to let constant address space to be 4 to match nvptx.
However for now we will make it 2 to avoid unnecessary work in FE/BE/devlib
about intrinsics returning constant pointers.

Differential Revision: https://reviews.llvm.org/D31770

llvm-svn: 299690