[clang] Introduce paged vector (PR #66430)

[David Blaikie via cfe-commits]

dwblaikie wrote:

> This only affects builds with GCC.

My understanding was that we basically only cared about performance of clang on clang - that we expect people who want an efficient clang to bootstrap?

Could `always_inline` the function, but those sort of annotations would get out of date/bitrot.

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