[Index] Report specialization bases as references when IndexImplicitInstantiation is true

Summary:
    template <typename T> struct B {};
    template <typename T> struct D : B<T> {}; // `B` was not reported as a reference

This patch fixes this.

Reviewers: akyrtzi, arphaman, devnexen

Reviewed By: devnexen

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52331

llvm-svn: 342831