[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~. Looks like the instruction count increased and the branching increased. Could it be the indexing operation:
> 
> https://github.com/llvm/llvm-project/blob/f99bd296108756e9824b179761fb5a40807af66f/llvm/include/llvm/ADT/PagedVector.h#L82-L95
> 
> Would adding an `LLVM_UNLIKELY` in the if-stmt condition help us?

@ktf could you take a look at this?

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