R600: Add definition for flat address space ID.

Use 4 since that's probably what it will be for spir.
Move ADDRESS_NONE to the end to keep the constant_buffer_* values
unchanged, since apparently a bunch of r600 tests use those directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209463 91177308-0d34-0410-b5e6-96231b3b80d8