[clang] [libc++] Prevent calling the projection more than three times (PR #66315)

[Jocelyn Castellano via cfe-commits]

pandaninjas wrote:

Any idea why the CI is failing? It mentions line 86 not being constexpr because of assert, but it doesn't seem like I've done anything which should affect that...

https://github.com/llvm/llvm-project/pull/66315