[clang] [ExposeObjCDirect] Adding a flag to expose symbols with objc_direct (PR #170616)

[Peter Rong via cfe-commits]

DataCorrupted wrote:

TODO: We switch to `-fobjc-direct-caller-thunks` or `-fobjc-direct-nil-check-thunk`. Would love to hear ppl's opinions if any. Naming things are hard.

I will do that after later PRs are merged into this branch, otherwise there will be a lot of conflicts to resolve (later PRs already used the flag in the test)

cc @kyulee-com 

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