glsl: When constructing a variable with an interface type, set interface_type

glsl: When constructing a variable with an interface type, set interface_type

Ever since the addition of interface blocks with instance names, we have
had an implicit invariant:

    var->type->is_interface() ==
        (var->type == var->interface_type)

The odd use of == here is intentional because !var->type->is_interface()
implies var->type != var->interface_type.

Further, if var->type->is_array() is true, we have a related implicit
invariant:

    var->type->fields.array->is_interface() ==
        (var->type->fields.array == var->interface_type)

However, the ir_variable constructor doesn't maintain either invariant.
That seems kind of silly... and I tripped over it while writing some
other code.  This patch makes the constructor do the right thing, and it
introduces some tests to verify that behavior.

v2: Add general-ir-test to .gitignore.  Update the description of the
ir_variable invariant for arrays in the commit message.  Both suggested
by Paul.

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