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

[via cfe-commits]

cor3ntin wrote:

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

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