[clang] Introduce paged vector (PR #66430)

[Giulio Eulisse via cfe-commits]

ktf wrote:

@zygoloid @kuhar thank you for the detailed comments. I am still reviewing some of them, in particular the one of @zygoloid  about the equality operator does not seem to work out of the box, so I need a bit more time thinking about it. I pushed a new version with the current state and resolved all the comments I think I have fixed.

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