Implement CoerceUnsized for rc::Weak

[remram44]

This is a simple addition, shouldn't change behavior.

Fixes #26704

I don't know if the coercion for Rc is tested, if it is this probably needs the same test with Weak.

[rust-highfive]

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[eddyb]

Could probably do the same thing for arc::Weak. Tests are in src/test/run-pass/dst-coerce-rc.rs.

[remram44]

I feel like maybe dst-coerce-rc.rs is more about testing the CoerceUnsized feature than checking it is implemented for the right types, but here you go.

[eddyb]

Thanks! @bors r+

[bors]

📌 Commit 3278e79 has been approved by eddyb

[bors]

⌛ Testing commit 3278e79 with merge c4b4f07...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt