For class template implicit instantiation, also update its location to point

For class template implicit instantiation, also update its location to point
to the pattern template that it came from, otherwise we had this situation:

template <typename T1, typename T2>
struct S {
};

template <typename T>
struct S<T, int> {
};

void f() {
  S<int, int> s; // location of declaration "S<int, int>" was of "S<T1, T2>" not "S<T, int>"
}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150290 91177308-0d34-0410-b5e6-96231b3b80d8