Enable passing binAssign expressions as ref/out arguments.

[kinke]

Fixes runnable/testassign.d:

void test12211()
{
    int a = 0;
    void foo(ref int x)
    {
        assert(x == 10);
        assert(&x == &a);
        x = 3;
    }
    foo(a = 10);
    assert(a == 3);
    foo(a += 7);
    assert(a == 3);
...
}

Previously, in foo(a += 7) a new lvalue based on the rvalue binAssign result was passed to foo() instead of the nested lvalue a.

[kinke]

This is unrelated, but caused a segfault while debugging with -vv. DeclarationExp::type may be null for current front-end (see https://github.com/ldc-developers/ldc/blob/merge-2.067/dmd2/expression.c#L1556).