[clang] [llvm] Recover performance loss after PagedVector introduction (PR #67972)

[Giulio Eulisse via cfe-commits]

ktf wrote:

I have updated the branch and I think the two changes in this PR are in any case good. However it's my understanding that the performance regression was actually due to some less aggressive inlining of the new code. I have no idea how to proceed for that though.

https://github.com/llvm/llvm-project/pull/67972