glsl2: Extend ir_constant to store constant arrays, and generate them.

glsl2: Extend ir_constant to store constant arrays, and generate them.

Since GLSL permits arrays of structures, we need to store each element
as an ir_constant*, not just ir_constant_data.

Fixes parser tests const-array-01.frag, const-array-03.frag,
const-array-04.frag, const-array-05.frag, though 03 and 04 generate the
wrong code.