glsl/sso: Add parser and AST-to-HIR support for separate shader object layouts

glsl/sso: Add parser and AST-to-HIR support for separate shader object layouts

GL_ARB_separate_shader_objects adds the ability to specify location
layouts for interstage inputs and outputs.

In addition, this extension makes 'in' and 'out' generally available for
shader inputs and outputs.  This mimics the behavior of
GL_ARB_explicit_attrib_location.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>