This is another work in progress commit, building on D3832 toward implementing
the design described in the ticket.
Wire up those callbacks in HscMain. The callbacks are not listened to anywhere
at this stage, so no behaviour should change.
|No Unit Test Coverage|
|Build 32004: [GHC] Linux/amd64: Patch building|
|Build 32003: [GHC] OSX/amd64: Continuous Integration|
|Build 32002: [GHC] Windows/amd64: Continuous Integration|
|Build 32001: arc lint + arc unit|