mesa/glspirv: Add struct gl_spirv_module

mesa/glspirv: Add struct gl_spirv_module

v2: * Make the SPIR-V module struct part of a larger gl_shader_spirv_data
    struct that will be introduced later, and don't reference it directly
    in gl_shader. (Eduardo Lima)
    * Readability improvements (Ian Romanick)

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