`<filesystem>`: Preallocate memory in `path::operator/`

[xenu]

Now, in the typical case, it will do at most a single allocation.

It's a hot path in my program, and a cursory GitHub search showed that path::operator/ is indeed commonly used.

[xenu]

@microsoft-github-policy-service agree

[StephanTLavavej]

Thank you! :heart_eyes_cat: Apologies for the (somewhat holiday-related) delay in getting this reviewed. I've pushed a conflict-free merge with main, followed by changes to address previous feedback, followed by changes for things I noticed, including test coverage. The logic looks solid (the only thing that needed a small patch was _Right being empty, and even that was probably technically well-defined due to basic_string null termination) - I really appreciate how you handled the strange corner cases.

We have a semi-manual process for simultaneously merging PRs to the GitHub and MSVC-internal repos. To save time, we batch up PRs, so your PR will be part of the next batch (possibly tomorrow but more likely next week).

[xenu]

Sorry for ignoring the review comments, I got busy with other stuff and then I forgot about this PR.

I've pushed a conflict-free merge with main, followed by changes to address previous feedback, followed by changes for things I noticed, including test coverage.

Thank you!

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for optimizing this important function, and congratulations on your first microsoft/STL commit! :rocket: :heart_eyes_cat: :tada:

This is expected to ship in VS 2022 17.10 Preview 2.