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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> to forward some attributes only when they are applicable

One question is whether the reflection proposal would address something like this. If it does, it might make more sense to invest in that than a custom solution. 

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