Structs with different names but identical members are treated as
ambiguous by the native HLSL compiler when looking up user-defined
functions. Add the struct name to the function name to work around
this limitation.

BUG=chromium:731324
TEST=angle_end2end_tests

Change-Id: Ie80ac0f1374bc5ac05dfebef3f94e2da7cdfc581
Reviewed-on: https://chromium-review.googlesource.com/558929
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>