Allow Operator Arguments

When resolving an operator list element reference, resolve all
operator operands and try to fold the operator first.  This allows the
operator to collapse to a list which may then be indexed.

Before, it was not possible to do this:
class D<int a, int b> { ... }
class C<list<int> A> : D<A[0], A[1]>;
class B<list<int> b> : C<!foreach(...,b)>;

Now it is.

llvm-svn: 141101