ESSL3 makes it possible to combine invariant with several more different
qualifiers. To avoid combinatorial explosion of the qualifier enum, track
invariance with a separate boolean.

TEST=WebGL conformance tests, angle_unittests
BUG=angleproject:987

Change-Id: I0c6629e5ca2ded06db9ac9e5bab2fb6480299a5a
Reviewed-on: https://chromium-review.googlesource.com/267662Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>