Instead of setting variable information in both CollectVariables and
the GetVariableTraverser helper class it uses, keep all of this
functionality in CollectVariables. A single helper function handles
setting variable information that doesn't depend on variable type, and
the rest is done in "record" functions that are implemented for each
variable type.

This removes templates from the code, making it leaner and easier to
understand, and will help with implementing future features like
adding binding and location layout qualifiers for uniforms.

BUG=angleproject:1442
TEST=angle_unittests, angle_end2end_tests,
     dEQP-GLES2.functional.shaders.*

Change-Id: I79148b7b3fa9cb46634a22bdcc9ce0c04f970384
Reviewed-on: https://chromium-review.googlesource.com/446838
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>