[llvm] Introduce paged vector (PR #66430)

[Giulio Eulisse via llvm-commits]

ktf wrote:

Ping @zygoloid. I have addressed the last 6 comments. I hope I did not miss any this time.
Ping @vgvassilev. Could you do one last pass as well and commit? I do not think I have commit rights. I also checked with my ROOT build and everything works as before and the memory gain is still there.

https://github.com/llvm/llvm-project/pull/66430