glsl: Add support for the framebuffer fetch layout(noncoherent) qualifier.

This allows the application to request framebuffer fetch coherency
with per-fragment output granularity.  Coherent framebuffer fetch
outputs (which is the default if no qualifier is present for
compatibility with older versions of the EXT_shader_framebuffer_fetch
extension) will have ir_variable_data::memory_coherent set to true.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>

src/compiler/glsl/ast.h

@@ -635,6 +635,11 @@ struct ast_type_qualifier {
          unsigned bound_sampler:1;
          unsigned bound_image:1;
          /** \} */
+
+         /** \name Layout qualifiers for GL_EXT_shader_framebuffer_fetch_non_coherent */
+         /** \{ */
+         unsigned non_coherent:1;
+         /** \} */
       }
       /** \brief Set of flags, accessed by name. */
       q;