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

[Peter Rong via cfe-commits]

DataCorrupted wrote:

@rjmccall I believe this feature to expose objc_direct symbols is read for review. Also tagging @MadCoder  since you are the original author, @ahatanak as you helped draft the [first version of the RFC](https://docs.google.com/document/d/16CsNCA2OKWkUM_LCw_qabTcMtPiq9ODVkZHALlDvzR8)

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