[clang] Introduce paged vector (PR #66430)

[Giulio Eulisse via cfe-commits]

ktf wrote:

@zygoloid @vgvassilev Ping. I think I have addressed all of your concerns. AFAICT, the only things I deliberately did not do are:

* Fix the comment as suggested in https://github.com/llvm/llvm-project/pull/66430#discussion_r1338111904. Unless I misread the code, I think the current comment is correct.
* I kept InvalidPage as a constant, even if it's currently nullptr. I have some evolution in mind where it might become useful to change it back to -1. Unless there is strong preference, I would leave it as it is.
* I did not change any other std::vectors to PagedVectors, in particular I did no change those suggested in https://github.com/llvm/llvm-project/pull/66430#discussion_r1337354442 because I cannot prove to myself I actually gain from memory from the change. I suggest we address those changes in a separate PR.

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