[clang] [clang] Implement a bitwise_copyable builtin type trait. (PR #86512)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

A few comments from me to move this review forward.
The major question I have is about the tests and how reference members should affect the results of that trait.

https://github.com/llvm/llvm-project/pull/86512