glsl: Don't allow unnamed interface blocks to redeclare variables.

glsl: Don't allow unnamed interface blocks to redeclare variables.

Note: some limited amount of redeclaration is actually allowed,
provided the shader is redeclaring the built-in gl_PerVertex interface
block.  Support for this will be added in future patches.

Fixes piglit tests
spec/glsl-1.50/compiler/unnamed-interface-block-elem-conflicts-with-prev-{block-elem,global}.vert.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>