When an attribute, a uniform or a varying isn't used, we now remove
their layout and in/out qualifiers so that the shader can still refer
to these var names.

Bug: angleproject:2456
Change-Id: I5f1241d91bd46f663750adfab2562ef87ce69ae5
Reviewed-on: https://chromium-review.googlesource.com/1014009
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>