C++: Fix resolving of templated partial specialization

Use-case:
template<class T> struct t {};

template<class> struct s { float f; };
template<class X> struct s<t<X>> { int i; };

void f()
{
    s<t<char>> var;
    var.i; // i not highlighted
}

Task-number: QTCREATORBUG-14034
Change-Id: I5d00bc3247352fca4af4c41a47c208ec3e193c8e
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>