[llvm] [WIP][ADT] Avoid slow size queries on append (PR #136365)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

@nikic How can I reproduce your compile time benchmarks locally, or find some close enough proxy? Would running opt on a large .bc file under perf be close enough? I think this should be the script used to produce the data: https://github.com/nikic/llvm-compile-time-tracker/blob/master/timeit.sh.

https://github.com/llvm/llvm-project/pull/136365