The shader validation now does the same checks for qualifier
combinations regardless of if a declaration is empty, as is specified.
Some of these checks used to be in singleDeclarationErrorCheck, and
have now been moved to a new function declarationQualifierErrorCheck.
The other parts of singleDeclarationErrorCheck are under a renamed
nonEmptyDeclarationErrorCheck.

The patch also contains another related cleanup: Unnecessary symbol
nodes won't be created for empty declarations any more.

BUG=angleproject:2020
TEST=angle_unittests

Change-Id: I1c864a5e151c52703926d8c550450b2561bfcbb2
Reviewed-on: https://chromium-review.googlesource.com/493227
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>