[clang] Introduce paged vector (PR #66430)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I feel like it looks better to touch https://llvm.org/docs/LangRef.html to introduce new ADT and tell what is the benefit (pros and cons) of the new data structure.

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