[analyzer][UninitializedObjectChecker] Explicit namespace resolution for inherited data members

For the following example:

  struct Base {
    int x;
  };

  // In a different translation unit

  struct Derived : public Base {
    Derived() {}
  };

For a call to Derived::Derived(), we'll receive a note that
this->x is uninitialized. Since x is not a direct field of Derived,
it could be a little confusing. This patch aims to fix this, as well
as the case when the derived object has a field that has the name as
an inherited uninitialized data member:

  struct Base {
    int x; // note: uninitialized field 'this->Base::x'
  };

  struct Derived : public Base {
    int x = 5;
    Derived() {}
  };

Differential Revision: https://reviews.llvm.org/D50905

llvm-svn: 340272