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

[Kinuko Yasuda via cfe-commits]

kinu wrote:

Would the alternative that is discussed here mean we want to forward some attributes only when they are applicable, something like `clang::forwad_lifetimebound`?

Regardless, I think it'd be also good to agree on the use cases we want to support in a concrete code snippet (that should also answer to @Xazax-hun 's question).  There could also be a question of whether we really should try to support variadic templates in the current lifetime-bound too, because we're aware that the current semantic itself has a limitation.


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