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

[Haojian Wu via cfe-commits]

hokein wrote:

> It would be nice if you could share some code snippets how exactly this would be used in practice.

Some simple examples from Abseil. When using `lifetime_capture` in the `insert` method, we currently have two overloads:  
- [one overload](https://github.com/abseil/abseil-cpp/blob/7971b4aabde5d326bc18dd7faa6fba6c891aa40a/absl/container/internal/raw_hash_set.h#L2904) is annotated (for view types);
- [the other overload](https://github.com/abseil/abseil-cpp/blob/7971b4aabde5d326bc18dd7faa6fba6c891aa40a/absl/container/internal/raw_hash_set.h#L2894) is unannotated (for non-view types);

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