Rename 'actor class' -> 'actor'

This patch softly updates the spelling of actors from `actor class` to
`actor`. We still accept using `actor` as a modifying attribute of
class, but emit a warning and fix-it to make the change.

One of the challenges that makes this messier is that the modifier list
can be in any order. e.g, `public actor class Foo {}` is the same as
`actor public class Foo {}`.

Classes have been updated to include whether they were explicitly
declared as an actor. This change updates the swiftmodule serialization
version number to 0.591. The additional bit only gets set of the class
declaration was declared as an actor, not if the actor was applied as an
attribute. This allows us to correctly emit `actor class` vs `actor`
emitting the code back out.

lib/Serialization/Serialization.cpp

@@ -3397,6 +3397,7 @@ public:
                             contextID.getOpaqueValue(),
                             theClass->isImplicit(),
                             theClass->isObjC(),
+                            theClass->isExplicitActor(),
                             mutableClass->inheritsSuperclassInitializers(),
                             mutableClass->hasMissingDesignatedInitializers(),
                             S.addGenericSignatureRef(