- add two createImplementationStubCore methods to StubUtility2Core
  corresponding to the jdt.ls versions of createImplementationStub
  methods in their copied/modified version of StubUtility2
- move getImplementationModifiers() method to StubUtility2Core from
  StubUtility2 and make public so it can be shared
- ditto for createParameters(), createTypeParameters(), and
  createThrownExceptions()

Change-Id: I3a68682d3357baa6897f8cc144d1dd4d1d2d2eff
Signed-off-by: Jeff Johnston <jjohnstn@redhat.com>