Fix Issue 11681 - std.datetime.IntervalRange.opAssign with non-ref parameter is required

https://d.puremagic.com/issues/show_bug.cgi?id=11681

This change enables an rvalue to assigned to IntervalRange/PosInfIntervalRange/NegInfIntervalRange.
```d
auto interval1 = NegInfInterval!Date(Date(2013, 1, 1));
auto interval2 = NegInfInterval!Date(Date(2014, 2, 1));

auto range1 = interval1.bwdRange(a => a - 1.days);
auto range2 = interval2.bwdRange(a => a - 1.days);

//  range1 = range2; // already possible
range1 = range2.save; // ok
range1 = interval2.bwdRange(a => a - 2.days); // ok
```