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

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>