mesa/glspirv: Add struct gl_shader_spirv_data

mesa/glspirv: Add struct gl_shader_spirv_data

This is a per-shader structure holding the SPIR-V data associated with the
shader (binary module, specialization constants and entry-point).

This is needed because both gl_shader and gl_linked_shader need to share this
data. Instead of copying the data, we pass a reference to it upon program
linking. That's why it is reference-counted.

This struct is created and associated with the shader upon calling
glShaderBinary(), then subsequently filled up by the call to
glSpecializeShaderARB().

v2: Readability improvements (Ian Romanick)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>