ESSL 1.00.17 section 6.1 forbids this. Returning arrays was already
covered by the checks that disallow declaring array types in ESSL
1.00.

BUG=angleproject:1015
TEST=dEQP-GLES2.functional.shaders.functions.* (2 new tests pass)
     dEQP-GLES3.functional.shaders.functions.* (no regression)

Change-Id: Iaffa1631c0c940afb57819221e7e8603b2305021
Reviewed-on: https://chromium-review.googlesource.com/352920Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>