Fix #20888 - Compiler spits out implicit conversion technobabble when a return statement doesn't match the return type of a function

[0xEAB]

• Testsuite has not been updated yet.
• Feel free to suggest even better error messages.

[dlang-bot]

Thanks for your pull request, @0xEAB!

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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 + dmd#20890"