[ABI] Set a bit in the class context descriptor for actor types.

Allow runtime metadata queries to determine if a "class" (in the
runtime) is actually an actor by adding a bit to the class context
descriptor's type-specific kind flags.

Implements rdar://77073762.

Author: DougGregor

include/swift/ABI/Metadata.h

@@ -4290,6 +4290,10 @@ class TargetClassDescriptor final
     return FieldOffsetVectorOffset;
   }
 
+  bool isActor() const {
+    return this->getTypeContextDescriptorFlags().class_isActor();
+  }
+
   bool isDefaultActor() const {
     return this->getTypeContextDescriptorFlags().class_isDefaultActor();
   }

include/swift/ABI/MetadataValues.h

@@ -1395,6 +1395,11 @@ class TypeContextDescriptorFlags : public FlagSet<uint16_t> {
 
     // Type-specific flags:
 
+    /// Set if the class is an actor.
+    ///
+    /// Only meaningful for class descriptors.
+    Class_IsActor = 7,
+
     /// Set if the class is a default actor class.  Note that this is
     /// based on the best knowledge available to the class; actor
     /// classes with resilient superclassess might be default actors
@@ -1485,6 +1490,9 @@ class TypeContextDescriptorFlags : public FlagSet<uint16_t> {
   FLAGSET_DEFINE_FLAG_ACCESSORS(Class_IsDefaultActor,
                                 class_isDefaultActor,
                                 class_setIsDefaultActor)
+  FLAGSET_DEFINE_FLAG_ACCESSORS(Class_IsActor,
+                                class_isActor,
+                                class_setIsActor)
 
   FLAGSET_DEFINE_FIELD_ACCESSORS(Class_ResilientSuperclassReferenceKind,
                                  Class_ResilientSuperclassReferenceKind_width,

lib/IRGen/GenMeta.cpp

@@ -1620,6 +1620,9 @@ namespace {
         if (MetadataLayout->hasResilientSuperclass())
           flags.class_setHasResilientSuperclass(true);
 
+        if (getType()->isActor())
+          flags.class_setIsActor(true);
+
         if (getType()->isDefaultActor(IGM.getSwiftModule(),
                                       ResilienceExpansion::Maximal))
           flags.class_setIsDefaultActor(true);

test/IRGen/async/default_actor.swift

@@ -4,9 +4,9 @@
 // REQUIRES: concurrency
 
 // CHECK: @"$s13default_actor1ACMn" = hidden constant
-//   0x81000050: 0x01000000 IsDefaultActor
-//   0x81010050: the same, but using a singleton metadata initialization
-// CHECK-SAME: i32 {{-2130706352|-2130640816}},
+//   0x81800050: 0x01800000 IsActor + IsDefaultActor
+//   0x81810050: the same, but using a singleton metadata initialization
+// CHECK-SAME: i32 {{-2122317744|-2122252208}},
 
 import resilient_actor