GDScript: Consolidate behavior for assigning enum types

This makes sure that assigning values to enum-typed variables are
consistent. Same enum is always valid, different enum is always
invalid (without casting) and assigning `int` creates a warning
if there is no casting.

There are new test cases to ensure this behavior doesn't break in
the future.

modules/gdscript/gdscript_parser.cpp

@@ -3737,8 +3737,6 @@ String GDScriptParser::DataType::to_string() const {
 		}
 		case ENUM:
 			return enum_type.operator String() + " (enum)";
-		case ENUM_VALUE:
-			return enum_type.operator String() + " (enum value)";
 		case UNRESOLVED:
 			return "<unresolved type>";
 	}