[ABI] Add a mangled superclass type to class context descriptors.

This allows us to recover the complete superclass type from metadata.
Currently unused, but we want to stub out the space in the ABI.

Author: DougGregor

include/swift/ABI/Metadata.h

@@ -3683,6 +3683,10 @@ class TargetClassDescriptor final
       .class_getResilientSuperclassReferenceKind();
   }
 
+  /// The type of the superclass, expressed as a mangled type name that can
+  /// refer to the generic arguments of the subclass type.
+  TargetRelativeDirectPointer<Runtime, const char> SuperclassType;
+
   union {
     /// If this descriptor does not have a resilient superclass, this is the
     /// negative size of metadata objects of this class (in words).

lib/IRGen/GenMeta.cpp

@@ -1589,6 +1589,15 @@ namespace {
     }
 
     void addLayoutInfo() {
+
+      // TargetRelativeDirectPointer<Runtime, const char> SuperclassType;
+      if (auto superclassType = getType()->getSuperclass()) {
+        B.addRelativeAddress(IGM.getTypeRef(superclassType->getCanonicalType(),
+                                            MangledTypeRefRole::Metadata));
+      } else {
+        B.addInt32(0);
+      }
+
       auto properties = getType()->getStoredProperties();
 
       // union {

test/IRGen/class_metadata.swift

@@ -50,8 +50,8 @@ class B : A {}
 // CHECK-SAME: i32 {{.*}} [[B_NAME]]
 //   Metadata access function.
 // CHECK-SAME: i32 {{.*}} @"$s14class_metadata1BCMa"
-//   Superclass.
-// CHECK-SAME: @"$s14class_metadata1ACMn"
+//   Superclass type.
+// CHECK-SAME: @"symbolic \01____ 14class_metadata1AC"
 //   Negative size in words.
 // CHECK-SAME: i32 2,
 //   Positive size in words.
@@ -88,8 +88,8 @@ class C<T> : B {}
 // CHECK-SAME: i32 {{.*}} [[C_NAME]]
 //   Metadata access function.
 // CHECK-SAME: i32 {{.*}} @"$s14class_metadata1CCMa"
-//   Superclass.
-// CHECK-SAME: i32 {{.*}} @"$s14class_metadata1BCMn"
+//   Superclass type.
+// CHECK-SAME: @"symbolic \01____ 14class_metadata1BC"
 //   Negative size in words.
 // CHECK-SAME: i32 2,
 //   Positive size in words.
@@ -141,16 +141,16 @@ class D : E {}
 
 // CHECK:      [[D_NAME:@.*]] = private constant [2 x i8] c"D\00"
 // CHECK-LABEL: @"$s14class_metadata1DCMn" =
-//   Flags. 0x4200_0050 == HasOverrideTable | IndirectSuperclass | Unique | Class
-// CHECK-SAME: <i32 0x4200_0050>,
+//   Flags. 0x4200_0050 == HasOverrideTable | Unique | Class
+// CHECK-SAME: <i32 0x4000_0050>,
 //   Parent.
 // CHECK-SAME: i32 {{.*}} @"$s14class_metadataMXM"
 //   Name.
 // CHECK-SAME: i32 {{.*}} [[D_NAME]]
 //   Metadata access function.
 // CHECK-SAME: i32 {{.*}} @"$s14class_metadata1DCMa"
-//   Superclass.
-// CHECK-SAME: @"got.$s14class_metadata1ECMn.1"
+//   Superclass type.
+// CHECK-SAME: @"symbolic \01____ 14class_metadata1EC"
 //   Negative size in words.
 // CHECK-SAME: i32 2,
 //   Positive size in words.
@@ -174,3 +174,7 @@ class D : E {}
 // CHECK-SAME: @"$s14class_metadata1DCACycfC"
 // CHECK-SAME: }>, section
 class E {}
+
+// CHECK-LABEL: @"$s14class_metadata1FCMn" =
+// CHECK-SAME: @"symbolic \01____yq_G 14class_metadata1CC"
+class F<T, U> : C<U> { }

test/IRGen/class_resilience.swift

@@ -43,8 +43,6 @@
 // CHECK-SAME:   @"$s16class_resilience14ResilientChildCMa"
 // --       field descriptor:
 // CHECK-SAME:   @"$s16class_resilience14ResilientChildCMF"
-// -- superclass:
-// CHECK-SAME:   @"got.$s15resilient_class22ResilientOutsideParentCMn"
 // -- metadata bounds:
 // CHECK-SAME:   @"$s16class_resilience14ResilientChildCMo"
 // --       metadata positive size in words (not used):
@@ -55,6 +53,8 @@
 // CHECK-SAME:   i32 1,
 // --       field offset vector offset:
 // CHECK-SAME:   i32 3,
+// -- superclass:
+// CHECK-SAME:   @"got.$s15resilient_class22ResilientOutsideParentCMn"
 // --       singleton metadata initialization cache:
 // CHECK-SAME:   @"$s16class_resilience14ResilientChildCMl"
 // --       resilient pattern: