[metadata prespecialization] Create enum records.

Extracted implementation of SpecializedGenericStructMetadataBuilder into
SpecializedGenericNominalMetadataBuilderBase, a CRTP with a template
template argument for the CRTP superclass and a template argument for
the implementation.  That new type is now subclassed by
SpecializedGenericStructMetadataBuilder.  Additionally, this new type is
also subclassed by the newly added SpecializedGenericEnumMetadataBuilder
which is responsible for build the prespecialization of generic enum
metadata.

rdar://problem/56960887

Author: nate-chandler

include/swift/ABI/Metadata.h

@@ -1426,6 +1426,32 @@ struct TargetEnumMetadata : public TargetValueMetadata<Runtime> {
     return *asWords;
   }
 
+  bool isCanonicalStaticallySpecializedGenericMetadata() const {
+    auto *description = getDescription();
+    if (!description->isGeneric())
+      return false;
+
+    auto *trailingFlags = getTrailingFlags();
+    if (trailingFlags == nullptr)
+      return false;
+
+    return trailingFlags->isCanonicalStaticSpecialization();
+  }
+
+  const MetadataTrailingFlags *getTrailingFlags() const {
+    auto description = getDescription();
+    auto flags = description->getFullGenericContextHeader()
+                     .DefaultInstantiationPattern->PatternFlags;
+    if (!flags.hasTrailingFlags())
+      return nullptr;
+    auto offset =
+        getGenericArgumentOffset() +
+        description->getFullGenericContextHeader().Base.getNumArguments() +
+        (hasPayloadSize() ? 1 : 0);
+    auto asWords = reinterpret_cast<const void *const *>(this);
+    return reinterpret_cast<const MetadataTrailingFlags *>(asWords + offset);
+  }
+
   static constexpr int32_t getGenericArgumentOffset() {
     return sizeof(TargetEnumMetadata<Runtime>) / sizeof(StoredPointer);
   }

lib/IRGen/EnumMetadataVisitor.h

@@ -62,6 +62,14 @@ template <class Impl> class EnumMetadataVisitor
            Target->getDeclaredTypeInContext()->getCanonicalType());
     if (strategy.needsPayloadSizeInMetadata())
       asImpl().addPayloadSize();
+
+    if (asImpl().hasTrailingFlags())
+      asImpl().addTrailingFlags();
+  }
+
+  bool hasTrailingFlags() {
+    return Target->isGenericContext() &&
+           IGM.shouldPrespecializeGenericMetadata();
   }
 };
 
@@ -86,6 +94,7 @@ class EnumMetadataScanner : public EnumMetadataVisitor<Impl> {
   void addGenericWitnessTable(GenericRequirement requirement) { addPointer(); }
   void addPayloadSize() { addPointer(); }
   void noteStartOfTypeSpecificMembers() {}
+  void addTrailingFlags() { addPointer(); }
 
 private:
   void addPointer() {