Fix a use of an imported InstId used where a local InstId is required

[dwblaikie]

Found by WIP validation for this type of issue ongoing in #5997

I'm not entirely sure how the one test update falls out of this change -
but it is from the same test that I originally reduced the problem from,
which is reassuring.

The reduced test case I investigated the issue with was this:
a.carbon:

library "lib";
interface I1(Other:! type) {
   let Result:! type;
}

b.carbon:

import library "lib";
class T1 { }
impl T1 as I1(Self) where .Result = Self { }

The SemIR dump diff looked like this:

89c89
<   %Main.import_ref.b6f = import_ref Main//lib, inst28 [no loc], unloaded
---
>   %Main.import_ref.b6f = import_ref Main//lib, inst27 [no loc], unloaded
96c96
<   %Main.import_ref.f7b: @I1.%I1.type (%I1.type.e87) = import_ref Main//lib, inst28 [no loc], loaded [symbolic = @I1.%Self (constants.%Self.c47)]
---
>   %Main.import_ref.f7b: @I1.%I1.type (%I1.type.e87) = import_ref Main//lib, inst27 [no loc], loaded [symbolic = @I1.%Self (constants.%Self.c47)]

Which is a difference, but given the inst28/inst27 don't appear
anywhere else than these two lines, it doesn't give a terribly
meaningful diff/story about what changed - but perhaps it's
sufficient...

Not sure if this test ^ is sufficiently more interesting than the diff
update already in this patch. If so, happy to add the above as a new
test case.

Open to ideas.

[zygoloid]

LGTM. Given that this will be tested once the Id checking work lands, I'm happy merging this without a more direct test.