The same function can't be declared twice in ESSL 1.00. In ESSL 3.00 this
is allowed.

A function prototype following the definition of that function is not
interpreted as redeclaration, and the shader compiler continues to allow
this.

BUG=angleproject:1067
TEST=angle_unittests,
     dEQP-GLES2.functional.shaders.functions.invalid.double_declare*

Change-Id: I6d2ddafd456d378d92839600f19069ad1cd19aff
Reviewed-on: https://chromium-review.googlesource.com/320082
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>