glsl: Add ir support for sample qualifier; adjust compiler and linker

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/glsl/ir_clone.cpp

@@ -52,6 +52,7 @@ ir_variable::clone(void *mem_ctx, struct hash_table *ht) const
    }
    var->read_only = this->read_only;
    var->centroid = this->centroid;
+   var->sample = this->sample;
    var->invariant = this->invariant;
    var->interpolation = this->interpolation;
    var->location = this->location;