[clang] [Clang] Add [[readnone, readonly, writeonly]] parameter attributes (PR #178429)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

How do we define these at the C++ language level?  Like, can a user actually tell if source-level assignment operator satisfies "writeonly"?  (For example, writing to a bitfield is a read.)

https://github.com/llvm/llvm-project/pull/178429