This data was previously stored entirely in the Impl level. Move
as much as possible to the GL level, using a read-only view in the
Impl level. Some information in D3D-specific, and should be stored
separately in the Impl.

This patch has a lot of refactoring that splits the D3D and GL info,
and moves as much validation as possible to the GL layer, where it
is shared between the back-ends.

Re-land with fix for dEQP unused uniforms. The fix involves storing
a local copy of all uniform data in the GL layer. This will also
let us validate sampler indexes during draw calls at the GL layer.

Re-re-land with a fix for multiply defined symbols on Clang.

BUG=angleproject:1123
TEST=end2end_tests,Canary WebGL (D3D11/GL), dEQP-GLES2+3.functional.uniform_api

Change-Id: I7b55e4964fbe64e0106c37928c894146937a2476
Reviewed-on: https://chromium-review.googlesource.com/298110Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>