This is done by declaring a temporary variable which is passed as the
array out parameter defined by ArrayReturnValueToOutParameter.
SeparateExpressionsReturningArrays takes care of transforming the rest of
the cases where a function call returns an array into form that
ArrayReturnValueToOutParameter can handle.

BUG=angleproject:971
TEST=WebGL 2 conformance tests, dEQP-GLES3.functional.shaders.arrays.*

Change-Id: I70c07712ba5cd91efb4c2e575ecc49b9ef71bfd7
Reviewed-on: https://chromium-review.googlesource.com/273111Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>