Pass std::string_view by value not by ref

[cbracken]

String views are small, cheap objects that fit in a couple registers, so
passing by value avoids pointer indirection and thus a memory load.

Fun related reading for future archaeologists:
https://quuxplusone.github.io/blog/2021/11/09/pass-string-view-by-value/

And a Windows-specific footnote:
https://quuxplusone.github.io/blog/2021/11/19/string-view-by-value-ps/

No test change since there is no semantic change.

Pre-launch Checklist

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• I read the Tree Hygiene wiki page, which explains my responsibilities.
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• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
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• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[Hixie]

test-exempt: refactor with no semantic change