Mark TypeDecls used in member initializers as referenced.

Without this, -Wunused-local-typedef would incorrectly warn on the two typedefs
in this program:

void foo() {
  struct A {};
  struct B : public A {
    typedef A INHERITED;
    B() : INHERITED() {}

    typedef B SELF;
    B(int) : SELF() {}
  };
}


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221765 91177308-0d34-0410-b5e6-96231b3b80d8