[5.9] Add diagnostic for wrong inheritance

[kimdv]

• Explanation: When there was a Python style inheritance the parser did show an "unexpected code" error. This will show a correct error and fix-it
• Scope: Swift parser and diagnostics
• Risk: Low
• Testing: Added test cases that converts Python style inheritance to Swift inheritance
• Issue: N/A
• Reviewer: @ahoppen on Add diagnostic for wrong inheritance #1450

[ahoppen]

Looking at this PR again, I’d prefer not to cherry-pick it to 5.9. The change seems potentially risky (because isAtPythonStyleInheritanceClause is not super trivial) and the use case is very marginal.