Function prototypes should not be allowed inside other functions
according to ESSL 3.00.4 section 4.2.4.

BUG=angleproject:1068
TEST=angle_unittests,
     dEQP-GLES*.functional.shaders.functions.invalid.local_function_proto*

Change-Id: I54160da4d49b92a6cd7cbee020e67733963d4e10
Reviewed-on: https://chromium-review.googlesource.com/320091
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>