spirv/nir: create nir variable for UBO/SSBO

spirv/nir: create nir variable for UBO/SSBO

Providing nir variables for UBO/SSBO it is not required for Vulkan,
but it is needed for OpenGL (ARB_gl_spirv), like for example, to
gather info from the UBO/SSBO while linking.

In opposite with most cases where the nir variables is created, here
the type assigned is the full type (not just the bare type). This is
needed because while linking using the nir shader we need the explicit
layout info (explicit stride, explicit offset, row_major, etc).

Also, we need to assign an interface type, used also on the OpenGL
linker if it is a UBO/SSBO. See ir_variable::is_in_buffer_block as
example.

v2: assign interface_type to be the variable type, not need to be
    arrayness (Timothy)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>