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

[via cfe-commits]

higher-performance wrote:

Just a friendly bump to figure out how to proceed here.

Does the `clang::lifetimebound_like` idea sound like a decent middle ground to move forward with? And if so, what do we anticipate the timeline for implementing it to be like? This would affect whether we move forward with workarounds in the meantime.

@ilya-biryukov

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