GLSL ES spec accepts the case where an initializer of a variable calls
a function with the same name as the variable. The HLSL compiler
doesn't accept that. Work around this limitation in the HLSL compiler
by disambiguating user-defined functions from variables with a
different prefix.

BUG=angleproject:2095
TEST=angle_end2end_test, angle_unittests

Change-Id: I41b32a3fcc6fd4c548e8dc3aa680d1b07fcf8719
Reviewed-on: https://chromium-review.googlesource.com/557872Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>