Introduce checking for ConcurrentValue conformance across actors.

When referring to an actor-isolated declaration from outside of the
actor, ensure that the types involved conform to the `ConcurrentValue`
protocol. Otherwise, produce a diagnostic stating that it is unsafe to
pass such types across actors.

Apply the same rule to local captures within concurrent code.

tools/swift-api-digester/swift-api-digester.cpp

@@ -1168,7 +1168,8 @@ public:
         DeclAttribute::canAttributeAppearOnDeclKind(DeclAttrKind::DAK_Available,
                                                     D->getDeclKind()) &&
         !D->getIntroducingVersion().hasOSAvailability() &&
-        !D->hasDeclAttribute(DeclAttrKind::DAK_AlwaysEmitIntoClient)) {
+        !D->hasDeclAttribute(DeclAttrKind::DAK_AlwaysEmitIntoClient) &&
+        !D->hasDeclAttribute(DeclAttrKind::DAK_Marker)) {
       D->emitDiag(D->getLoc(), diag::new_decl_without_intro);
     }
   }