Support use of an optional label for the first trailing closure

[karim (Karim Nassar)]

@barnard-b

Affected APIs would either have to update in sync with the label change release or face code breakage. Having to write DispatchQueue.main.async execute: { (even temporarily) would not be pleasant.

It’s difficult for sure. Unfortunately, the price of not being that strict is that we’ll be dealing with API inconsistency for the next decade or longer, which IMNSHO, is way worse.