[clang] [WIP][clang]: Implement a conditional lifetimebound_if builtin. (PR #125520)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> mean we want to forward some attributes only when they are applicable

yes, something like that. And it gets a little tricky, because it's not that we want to forward only attributes from one decl to another, it's many decls (forwarding function itself and its parameters) to many decls (function we're forwarding too and its parameters). If we start talking about `lifetime_capture_by()`, things get even more tricky because the attribute itself is parametrized by the the "target" and we should somehow be able to connect that "target" to something else in the context we are forwarding to.

https://github.com/llvm/llvm-project/pull/125520