glsl: Add parser/compiler support for std430 interface packing qualifier

glsl: Add parser/compiler support for std430 interface packing qualifier

v2:
- Fix a missing check in has_layout()

v3:
- Mention shader storage block in error message for layout qualifiers
  (Kristian).

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>