Allow Operator Arguments

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141101 91177308-0d34-0410-b5e6-96231b3b80d8