constify From impls for the Cow::Borrowed variant

[npmccallum]

This issue is a follow-up PR from #146067 with the controversial impls removed.

This is useful in at least two cases:

1. Generic interfaces which may be const.
2. Conditional compilation (&str vs Cow<str>).

[rustbot]

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[tgross35]

Anybody reviewing this, please take a look at #146067 (comment)