[record_use] Correctness: Record all loading units for const instances

[dcharkes]

Currently our compiler backends may shortcut and stop emitting a const instance if it already has seen it in one loading unit.

We should change the semantics so that all loading units are correctly supported.

• dart compiler emitting multiple loading units for const instances.
• integration tests for multiple loading units in Dart SDK for const instances.

I believe calls are already always recorded, but we don't have tests for it.

• integration test for multiple loading units in the Dart SDK for calls.

(Follow up of #3022)

(Orthogonal and related: #3092)