swift234

[PlaygroundTransform] Instrument mutations of inout vars by class methods.

+= is now a class method, which means the playground transform ignores it.  This
is undesirable, and we have ways to instrument methods that mutate inout
parameters the way += does.  I have just made it so that if we see a method call
and we can't instrument the mutation of the base of the method call, we still
instrument the inout argument if there is one.

Also added a test case.