Issue 9148 - 'pure' is broken

https://issues.dlang.org/show_bug.cgi?id=9148

A nested function inside `pure` function should also be typed as `pure`, and considered as weak purity function.
I think this is broken behaviour. I would suggest to indeed stop _incorrectly_ reporting the purity violation, but this can break code unless this is taken care of by e.g. adding attribute inference for local functions. I guess it is ok to merge this as is for now, because it can be fixed later.