This is a followup CL of
https://chromium-review.googlesource.com/366720. Unittests is added to
check invariant status does not leak across shaders.

This CL also moves mInvariantVaryings and mGlobalInvariant from
TSymbolTable to TSymbolTableLevel. So at the end of a compilation, the
levels pop, and the settings will be cleared and will not affect the
next compilation.

Change-Id: I1199fade7a637276ab149ab9a599621b9977298b
Reviewed-on: https://chromium-review.googlesource.com/370844
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>