glsl: Rename ir_constant::array_elements to ::const_elements

glsl: Rename ir_constant::array_elements to ::const_elements

The next patch will unify ::array_elements and ::components, so the
name ::array_elements wouldn't be appropriate.  A lot of things use
the names array_elements and components, so grepping for either is
pretty useless.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Elie Tournier <elie.tournier@collabora.com>