Do this by separating each array initialization into a declaration and an
assignment. Array assignment is already supported in HLSL output by replacing
it with a function call.

The functionality is tested by the struct array constructor tests in dEQP.

BUG=angleproject:941
TEST=dEQP-GLES3.functional.shaders.arrays.constructor.*

Change-Id: Ida84fc343b767bea8b2d04e91c60cb8197d39039
Reviewed-on: https://chromium-review.googlesource.com/266002Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>