glsl: Disallow primitive type layout qualifier on variables.

glsl: Disallow primitive type layout qualifier on variables.

This only makes any sense on the GS input or output layout declaration,
nowhere else.

Fixes the piglit tests:

  * spec/glsl-1.50/compiler/incorrect-in-layout-qualifiers-with-variable-declarations.geom
  * spec/glsl-1.50/compiler/incorrect-out-layout-qualifiers-with-variable-declarations.geom
  * spec/glsl-1.50/compiler/layout-fs-no-output.frag
  * spec/glsl-1.50/compiler/layout-vs-no-input.vert
  * spec/glsl-1.50/compiler/layout-vs-no-output.vert

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>