swift-api-digester: simplify some APIs by centralizing memory management. NFC (#5827)

Author: nkcsgexi

tools/swift-api-digester/DigesterEnums.def

@@ -29,7 +29,6 @@ NODE_KIND(Getter)
 NODE_KIND(Setter)
 NODE_KIND(Var)
 NODE_KIND(TypeAlias)
-NODE_KIND(Nil)
 
 NODE_ANNOTATION(Added)
 NODE_ANNOTATION(Removed)