glsl: Parse shared keyword for compute shader variables

glsl: Parse shared keyword for compute shader variables

v2:
 * Move shared parsing under storage qualifiers (tarceri)
 * Fail to compile if shared is used in non-compute shader (tarceri)
 * Use separate shared_storage bit for shared variables (tarceri)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>