Fix needless_doctest_main panic when doctest is invalid

[profetia]

Closes #8244
Closes #15041

This feels like a bug with the compiler, because the panic happens when Diag is getting unwinded. However, drop() is already called in .cancel() so this should not happen.

In this PR, I find a workaround to just call emit(), since the DiagCtxt here is just a io::sink, nothing will happen and the panic just goes away.

changelog: [needless_doctest_main] fix panic when doctest is invalid

[rustbot]

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