patch: clarify diagnostic when returned value doesn't match function return type

[Nick Lewycky via cfe-commits]

This simple-minded patch extends the case where we report "no viable
conversion from 'X' to 'Y'" to emit a more useful diagnostic "no viable
conversion from returned value of type 'X' to function return type 'Y'"
when used in that context.

In lieu of adding tests for this diagnostic explicitly, I've only updated
the tests where the patch changes their result.

Please review!

Nick
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