[CP][stable channel] Attach unboxing info to unreachable members

[a-siva]

commit(s) to merge: 3bef6cf

merge instructions: clean merge

What is the issue: #44563

What is the fix: Unreachable members could be used as interface targets of dispatch
table calls, so they should have correct unboxing metadata

Why cherrypick: The bug manifests as a core dump and could potentially happen in the field, the cost of evaluating crashes and root causing it is high, proactively fixing bugs that lead to crashes generally helps.

Risk: Low

Link to original issue(s): #44563
https://github.com/wm17/flutter_switch_segfault

/cc @kevmoo @mit-mit @whesse @athomas @vsmenon @franklinyow

[franklinyow]

Approved in SCRUM meeting.

[franklinyow]

For changelog.md:
This is a patch release that fixes a condition that result in a crash in the VM. (#44563 )

[athomas]

Merged to beta in 2b88e9d (2.12.0-133.7.beta).

Issue will remain open until merged to stable as well.

[athomas]

Merged to stable in 2ea318b (2.10.5).