glsl: Throw the required error when a case label is a non-constant.

glsl: Throw the required error when a case label is a non-constant.

It's not quite spelled out in the spec text, but the grammar indicates
that only constant values are allowed as switch() case labels (and
only constant values make sense, anyway).

Fixes piglit glsl-1.30/compiler/switch-statement/switch-case-uniform-int.vert.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>