Introduce paged vector (#66430)

The goal of the class is to be an (almost) drop in replacement for
SmallVector and std::vector when those are presized and filled later, as
it happens in SourceManager and ASTReader.

By doing so, sparsely accessed PagedVector can profit from reduced
memory footprint.

GitOrigin-RevId: 4ae51570806ba5c5fcabe6d6dcbe52e3a5d5453b
4 files changed