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

[Peter Rong via cfe-commits]

DataCorrupted 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.

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