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

[Erich Keane via cfe-commits]

erichkeane wrote:

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

Eh, it is pretty effectively 0.  If you subtract ones that are wrapped in '#if 0' or part of libcxx, the first two pages have like 3 entries on them.

> > 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?

I would just say 'document for a release' then remove, but that is a better idea.  perhaps `__has_builtin` is 0, plus some sort of `__HAD_DEPRECATED_IS_REFERENCEABLE` macro? 

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