glsl/linker: don't fail non static used inputs without matching outputs

If there is no Static Use of an input variable, the linker shouldn't
fail whenever there is no defined matching output variable in the
previous stage.

From page 47 (page 51 of the PDF) of the GLSL 4.60 v.5 spec:

  " Only the input variables that are statically read need to be
    written by the previous stage; it is allowed to have superfluous
    declarations of input variables."

Now, we complete this exception whenever the input variable has an
explicit location. Previously, 18004c33 ("glsl: fail when a
shader's input var has not an equivalent out var in previous") took
care of the cases in which the input variable didn't have an explicit
location.

v2: do the location based interface matching check regardless on
    whether it is a separable program or not (Ilia).

Fixes: 1aa5738e ("glsl: relax input->output validation for SSO programs")
Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>
Cc: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Cc: Tapani Pälli <tapani.palli@intel.com>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 3be05dd2)