Fixes Issue 8787 - Better diagnostic on non-final interface function with a body.
Fixes Issue 9312 - With diagnostic is wrong.
Merge pull request #1479 from AndrejMitrovic/Fix9312

Issue 9312 - With statement diagnostic is wrong
Merge pull request #1416 from AndrejMitrovic/Fix8787

Issue 8787 - Better diagnostic on non-final interface function with a body