When validating some shaders with out-of-bounds array indexes,
we would write the sanitized index into the global symbol table.
We would then overwrite a wrong value for the builtin. This
fixes the WebGL test extensions/webgl-draw-buffers-max-draw-buffers.

Also mark const on as many uses ConstantUnion as we can.

BUG=angleproject:993

Change-Id: I110efaf1b7b0158b08b704277e3bc2472437902c
Reviewed-on: https://chromium-review.googlesource.com/268962Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>