Do not allow to use type alias for method implementations in objfpc mode.

Currently the compiler allows to use type aliases to implement a method.

type
  TBird = class
    procedure Fly;
  end;
  TEagle = TBird; // alias

procedure TEagle.Fly;
begin
end;

or even

Interface

Type
   TSomeReallyAnnoyingIncrediblyLongClassName = Class
   Type
      TSomeReallyAnnoyingSubClass = class
         Procedure DoSomething;
      end;
   end;

implementation

Type
   T1 = TSomeReallyAnnoyingIncrediblyLongClassName.TSomeReallyAnnoyingSubClass;

procedure T1.DoSomething;

begin
end;

Delphi allows to parse this, so it should be allowed in mode delphi, but in mode objfpc this use of type aliases should be forbidden.