sumtype: fix canMatch for non-copyable ref return

[pbackus]

Handlers that return a non-copyable value by reference are now capable of successfully matching.

Fixes #10647

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10648"