The code previously failed to check for correctness of layout qualifiers
in case a declarator followed an empty declaration, like so:

layout(packed) uniform float, a;

Fix this by running all necessary declaration checks also for declarators
which follow an empty declaration.

structQualifierErrorCheck is merged into singleDeclarationErrorCheck.

TEST=angle_unittests, WebGL conformance tests
BUG=angleproject:969

Change-Id: Idcb0673e3bcf64087744ff0d260f51a7546f024a
Reviewed-on: https://chromium-review.googlesource.com/264812Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>