glsl/tests: Unit test vertex shader in / out with link_invalidate_variable_locations

glsl/tests: Unit test vertex shader in / out with link_invalidate_variable_locations

Validates:

  - ir_variable::explicit_location should not be modified.

  - If ir_variable::explicit_location is not set, ir_variable::location,
    ir_variable::location_frac, and
    ir_variable::is_unmatched_generic_inout must be reset to 0.

  - If ir_variable::explicit_location is set, ir_variable::location
    should not be modified.  ir_variable::location_frac, and
    ir_variable::is_unmatched_generic_inout must be reset to 0.
    Previous unit tests have shown that all non-generic inputs / outputs
    have explicit_location set.

v2: Split the link_invalidate_variable_locations interface change out to
a separate patch.  Remove the vertex_in_builtin_without_explicit and
vertex_out_builtin_without_explicit tests.  There was a lot of good
discussion about this on the mailing list to which I refer the
interested reader.  Both changes suggested by Paul.

    http://lists.freedesktop.org/archives/mesa-dev/2013-October/046652.html

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>