Test that fragment, vertex or compute shader exclusive builtins are
not accessible from another shader stage. This will protect against
regressions when refactoring the symbol table.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I50f97cbde3f1c5cb4584568e3008f1dab724b769
Reviewed-on: https://chromium-review.googlesource.com/941953
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>