Improve handling of declarator lists with empty declarations 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/264812 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>