This will enable more code reuse when handling sampler struct as
function arguments. We can add the sampler structs to the symbol table
stack when they are function arguments.

Bug: angleproject:2494
Change-Id: I9eeb1d3822e34cd43535e1b16a98864545755d22
Reviewed-on: https://chromium-review.googlesource.com/1117322
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>