Editorial: avoid duplicate SetFunctionName in {async,}generator methods

[bakkot]

#1668 introduced a DefineMethodProperty method for use when creating plain methods, generator methods, async methods, and async generator methods. In that PR it included a call to SetFunctionName, which was correspondingly factored out of the evaluation semantics of plain methods and async methods, but not generator methods or async generator methods. In the evaluation semantics for generator (and async generator) methods, there's another property (.prototype) which is added between the call to SetFunctionName and DefineMethodProperty, and since property creation order is observable dropping the first call and just letting it happen in DefineMethodProperty would be an observable change.

Instead, just remove SetFunctionName from DefineMethodProperty, and restore it to the evaluation semantics for plain and async methods. It doesn't really belong in DefineMethodProperty.

(I'm pretty sure I introduced this bug by accident rebasing #1668.)

This was definitely unintentional, and fails an assert (in SetFunctionName). So I'm calling it editorial.

Thanks to @devsnek for the catch.