Reduce allocations for path conversions on Windows

[AronParker]

Fixes #96297. Previously, UTF-8 to UTF-16 Path conversions on Windows unnecessarily allocate twice, as described in #96297. This commit fixes that issue.

[rust-highfive]

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[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @thomcc (or someone else) soon.

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[thomcc]

LGTM!

@bors r+

[bors]

📌 Commit 6cfdeaf has been approved by thomcc

[bors]

⌛ Testing commit 6cfdeaf with merge 64c5deb...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 64c5deb to master...

[rust-timer]

Finished benchmarking commit (64c5deb): comparison url.

Summary:

• Primary benchmarks: 🎉 relevant improvement found
• Secondary benchmarks: no relevant changes found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	0	0	1	0	1
mean2	N/A	N/A	-0.3%	N/A	-0.3%
max	N/A	N/A	-0.3%	N/A	-0.3%

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. number of relevant changes ↩

2. the arithmetic mean of the percent change ↩