Fix void_return rule to support async and async throws functions

[lo1tuma]

This fixes #4766. I’ve noticed there is also #4158 which rewrites the whole rule to use SwiftSyntax, which would be also sufficient to fix the issue. But this PR adds additional test cases for the async and async throws functions, so I think we can use this fix for now and consider #4158 as a long-term solution, which should hopefully also pass the new tests.

Fixes: #4766
Refs: #4158

[SwiftLintBot]

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[SimplyDanny]

Thanks! The fix looks good to me.