This change is pure refactoring and doesn't fix bugs related to
invariant declarations. Invariant declarations are supposed to accept
a list of identifiers, but this refactoring keeps the current behavior
of only accepting a single identifier in an invariant declaration.
When the bug will be fixed, the new TIntermInvariantDeclaration class
that now has only a single child node can be changed so that it may
have multiple children.

TIntermAggregate is still used for function calls, function
prototypes and function parameter lists.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I3e22092c87e1c06445fd7e123d9922c2fcb59428
Reviewed-on: https://chromium-review.googlesource.com/419415Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>