glsl: Error check redeclarations of gl_PerVertex.

glsl: Error check redeclarations of gl_PerVertex.

This patch verifies that:

- The gl_PerVertex input interface block may only be redeclared in a
  geometry shader, and that it may only be redeclared as gl_in[].

- The gl_PerVertex output interface block may only be redeclared in a
  vertex or geometry shader, and that it may only be redeclared as a
  non-array without an interface name.

- gl_PerVertex may not be redeclared as any other type of interface
  block (i.e. as a uniform interface block).

As a side-effect, the code now keeps track of what the previous
declaration of gl_PerVertex was--this will be needed in future
patches.

Fixes piglit tests:
- spec/glsl-1.50/compiler/gs-redeclares-pervertex-in-with-incorrect-name.geom
- spec/glsl-1.50/compiler/gs-redeclares-pervertex-out-as-array.geom
- spec/glsl-1.50/compiler/gs-redeclares-pervertex-out-with-instance-name.geom

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