[clang_delta] Fix offset in RemoveBaseClass pass

[mpflanzer]

Without the fix the opening brace of the base class's definition was copied to the derived class together with it's declarations. This resulted in invalid code. For instance

class B {
  int a;
};

class D : B {
  int c;
};

got transformed into

class D  {
  int c;
{
  int a;};

With the changes the result is

class D  {
  int c;

  int a;};

The same changes has also been made for C-Vise: https://github.com/marxin/cvise/blame/9f237d6619f08e85784e5356869f48f81678815f/clang_delta/RemoveBaseClass.cpp#L190

[chenyang78]

LGTM. Thanks, @mpflanzer !