[3.0 compat] Don't diagnose @escaping var-arg closures.
Var-arg closures are already escaping, so we typically want to issue
an error if @escaping is specified. To not do sure will confuse and
give a false impression on the un-annotated semantics and whether it
is needed. But, 3.0 shipped with a bug where we didn't consistently
apply the no-escape-by-default rule here, and thus it was semantically
meaningful (and needed).
In order to preserve source compatibility, albeit at the expense of
developers on versions 3.0.1 and later in the Swift 3 family, we
suppress the error if we see @escaping. Either way, this is a
relatively uncommon usage, so the confusion won't be too wide spread.
