[clang] Introduce paged vector (PR #66430)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> Looks like this causes a compile-time regression: https://llvm-compile-time-tracker.com/compare.php?from=abcaebfe3aacb13d46be5e949fd6ed9b4321e2f6&to=4ae51570806ba5c5fcabe6d6dcbe52e3a5d5453b&stat=instructions%3Au About 0.5% at `O0`.

It is probably the change in the `SourceManager.cpp` in non-modules mode that caused it. IIUC, this is a memory regression and probably comes by the larger page/slab that we allocate. What would be the best way to profile this?

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