[concurrency] Add a new method ActorIsolation::forActorInstanceSelf.

This occurs when working with ActorIsolation in SIL.

This lets us avoid needing to depend on the AST for getting ActorIsolation for
self parameters. Now, we can just create the actor isolation we need based off
of the decl that we have.

The code is based off of forActorInstanceSelf(ValueDecl *decl) along the path
where it just creates isolation based off of the decl's nominal type decl (which
is equivalent to what we are trying to do here).

(cherry picked from commit 04e8bad)

Author: gottesmm

include/swift/AST/ActorIsolation.h

@@ -111,6 +111,9 @@ class ActorIsolation {
 
   static ActorIsolation forActorInstanceSelf(ValueDecl *decl);
 
+  /// Create an ActorIsolation appropriate for a type that is self.
+  static ActorIsolation forActorInstanceSelf(NominalTypeDecl *decl);
+
   static ActorIsolation forActorInstanceParameter(NominalTypeDecl *actor,
                                                   unsigned parameterIndex) {
     return ActorIsolation(ActorInstance, actor, parameterIndex + 1);

lib/AST/Decl.cpp

@@ -11387,6 +11387,10 @@ ActorIsolation::forActorInstanceSelf(ValueDecl *decl) {
   return ActorIsolation(ActorInstance, dc->getSelfNominalTypeDecl(), 0);
 }
 
+ActorIsolation ActorIsolation::forActorInstanceSelf(NominalTypeDecl *selfDecl) {
+  return ActorIsolation(ActorInstance, selfDecl, 0);
+}
+
 NominalTypeDecl *ActorIsolation::getActor() const {
   assert(getKind() == ActorInstance ||
          getKind() == GlobalActor);