a predicate.
The new overload is used during macro expansion to find the nearest
syntax node that is either a `DeclSyntax` or a `ClosureExprSyntax`.
This avoids an issue where calling the overload that accepts a
specific type may find an outer, unrelated declaration or closure.
We introduce a new macro called #SwiftSettings that can be used in conjunction
with a new stdlib type called SwiftSetting to control the default isolation at
the file level. It overrides the current default isolation whether it is the
current nonisolated state or main actor (when -enable-experimental-feature
UnspecifiedMeansMainActorIsolated is set).
