In particular, this commit includes:
* Attempt to abstract access to FirSourceElement via FirModifier
* Add more visit functions to DeclarationCheckersDiagnosticComponent
* Add messages+factories for 4 modifier-related errors and warnings
* Introduce FirModifierChecker
Most of modifier diagnostic is expressed by REDUNDANT_MODIFIER, INCOMPATIBLE_MODIFIERS, REPEATED_MODIFIER, WRONG_MODIFIER_TARGET, WRONG_MODIFIER_PARENT.
A set of modifier diagnostics is not in use now (but not deleted yet).
Sealed classes can be derived only by their own inner classes or objects.
Their constructors cannot be called explicitly, so compiler knows all their descendants.
Incompatible modifier checks (final, abstract). Impossible with interface, object, enum.
A pack of tests provided.
FenstonSingel