[clang] [Clang] Remove __is_referenceable builtin (PR #123078)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

> Should we deprecate it in 20, remove in 21? _almost_ is not 0 https://github.com/search?q=__is_referenceable%28&type=code

What exactly would you like to do? Make `__has_builtin(__is_referenceable)` return 0 but still accept `__is_referenceable` for a release?

https://github.com/llvm/llvm-project/pull/123078