glsl: add support for unsized arrays in shader storage blocks

They only can be defined in the last position of the shader
storage blocks.

When an unsized array is used in different shaders, it might be
converted in different sized arrays, avoid get a linker error
in that case.

v2:
- Rework error condition and error messages (Timothy Arceri)

v3:
- Move OpenGL ES check to its own patch.

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>

src/glsl/ast_array_index.cpp

@@ -226,7 +226,8 @@ _mesa_ast_array_index_to_hir(void *mem_ctx,
              * by the linker.
              */
          }
-         else {
+         else if (array->variable_referenced()->data.mode !=
+                  ir_var_shader_storage) {
             _mesa_glsl_error(&loc, state, "unsized array index must be constant");
          }
       } else if (array->type->fields.array->is_interface()