[clang] [ObjCDirectPreconditionThunk] Adding a flag to with objc_direct symbols' prefix (PR #170616)

[Kyungwoo Lee via cfe-commits]

kyulee-com wrote:

> Oh I understand, I'm not too familiar with the stacked PR workflow, maybe that's where the confusion is coming from.
> 
> I have changed the names as suggested (cc @kyulee-com @rjmccall ), let me know if `-fobjc-direct-precondition-thunk` looks good to you.

If we’re confident, just turn the new ABI on by default and let people use`-fno-objc-direct-precondition-thunks` to temporarily opt out. Plan to remove the flag in the next release or so.
If not, start with opt-in as the above, then switch to default-on with an opt-out flag (and a warning), and eventually get rid of the flag altogether.
In either case, I think we should document these in place.

https://github.com/llvm/llvm-project/pull/170616