[6.0] Properly handle @objc thunks for generic classes and actors

[rjmccall]

Explanation: We were not properly handling generic arguments when emitting SIL for the Objective-C thunk associated with a generic class method. Generic methods are not generally allowed to be exposed to Objective-C, but an exception exists for methods that are only generic because of the type of self. It's likely that the mishandling at the SIL level would not cause real problems in simple generated code because the generic arguments are not passed independently in the actual ABI — they're just recovered from the object's isa metadata. However, it does cause assertion failures because the SIL is malformed, and it's possible that that could have other compile-time consequences.
Scope: Narrow; affects SIL generation of @objc method thunks in generic contexts
Issue: rdar://129187133
Original PR: #74375
Risk: Low
Testing: added new regression tests
Reviewer: Slava Pestov

[rjmccall]

@swift-ci Please test