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/ModuleFormat.h

@@ -56,7 +56,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 594; // @differentiable(reverse) attr
+const uint16_t SWIFTMODULE_VERSION_MINOR = 595; // Adding Actor class decls
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -1208,6 +1208,7 @@ namespace decls_block {
     DeclContextIDField,     // context decl
     BCFixed<1>,             // implicit?
     BCFixed<1>,             // explicitly objc?
+    BCFixed<1>,             // Explicitly actor?
     BCFixed<1>,             // inherits convenience initializers from its superclass?
     BCFixed<1>,             // has missing designated initializers?
     GenericSignatureIDField, // generic environment