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>"
}

llvm-svn: 150290