[llvm] [IR] Store Metadata attachments in vector (PR #189551)

[Alexis Engelke via llvm-commits]

aengelke wrote:

[stage2-O2 -0.37% instructions:u](https://llvm-compile-time-tracker.com/compare.php?from=146d604f9a74084c26df784d964578b81d672120&to=69b6ecbebd3c3bb9881e1c4248a171d3158e0cd2&stat=instructions:u)
[stage2-O3 -0.43% max-rss (significant)](https://llvm-compile-time-tracker.com/compare.php?from=146d604f9a74084c26df784d964578b81d672120&to=69b6ecbebd3c3bb9881e1c4248a171d3158e0cd2&stat=max-rss)

https://github.com/llvm/llvm-project/pull/189551