Change the pattern of generic class metadata instantiation.

Minimize the generic class metadata template by removing the
class header and base-class members.  Add back the set of
information that's really required for instantiation.
Teach swift_allocateGenericClass how to allocate classes without
superclass metadata.  Reorder generic initialization to establish
a stronger phase-ordering between allocation (the part that doesn't
really care about the generic arguments) and initialization (the
part that really does care about the generic arguments and therefore
might need to be delayed to handle metadata cycles).

A similar thing needs to happen for resilient class relocation.

Author: rjmccall

Diff for: include/swift/ABI/MetadataValues.h

@@ -1021,6 +1021,10 @@ class TypeContextDescriptorFlags : public FlagSet<uint16_t> {
     /// Only meaningful for class descriptors.
     Class_SuperclassReferenceKind = 12,
     Class_SuperclassReferenceKind_width = 2,
+
+    /// Whether the immediate class members in this metadata are allocated
+    /// at negative offsets.  For now, we don't use this.
+    Class_AreImmediateMembersNegative = 11,
   };
 
 public:
@@ -1037,6 +1041,9 @@ class TypeContextDescriptorFlags : public FlagSet<uint16_t> {
   FLAGSET_DEFINE_FLAG_ACCESSORS(Class_HasResilientSuperclass,
                                 class_hasResilientSuperclass,
                                 class_setHasResilientSuperclass)
+  FLAGSET_DEFINE_FLAG_ACCESSORS(Class_AreImmediateMembersNegative,
+                                class_areImmediateMembersNegative,
+                                class_setAreImmediateMembersNegative)
 
   FLAGSET_DEFINE_FIELD_ACCESSORS(Class_SuperclassReferenceKind,
                                  Class_SuperclassReferenceKind_width,

Diff for: include/swift/Remote/MetadataReader.h

@@ -233,7 +233,7 @@ class MetadataReader {
       return StoredPointer();
 
     auto classMeta = cast<TargetClassMetadata<Runtime>>(meta);
-    return classMeta->SuperClass;
+    return classMeta->Superclass;
   }
 
   /// Given a remote pointer to class metadata, attempt to discover its class
@@ -548,10 +548,10 @@ class MetadataReader {
       if (!classMetadata)
         return llvm::None;
 
-      if (!classMetadata->SuperClass)
+      if (!classMetadata->Superclass)
         return type->getGenericArgumentOffset(nullptr, nullptr);
 
-      auto superMetadata = readMetadata(classMetadata->SuperClass);
+      auto superMetadata = readMetadata(classMetadata->Superclass);
       if (!superMetadata)
         return llvm::None;
 
@@ -921,7 +921,7 @@ class MetadataReader {
         if (descriptorAddress || !skipArtificialSubclasses)
           return static_cast<StoredPointer>(descriptorAddress);
 
-        auto superclassMetadataAddress = classMeta->SuperClass;
+        auto superclassMetadataAddress = classMeta->Superclass;
         if (!superclassMetadataAddress)
           return 0;