Add Self- and SelfWitnessTable metadata sources to capture descriptors

These can show up in partial applications of functions with the
witness_method calling convention.

include/swift/Reflection/MetadataSource.h

@@ -182,6 +182,9 @@ class MetadataSource {
         return decodeGenericArgument(A, it, end);
       case 'P':
         return decodeParent(A, it, end);
+      case 'S':
+        ++it;
+        return A.template createSelf();
       default:
         return nullptr;
     }
@@ -315,6 +318,7 @@ public:
   }
 };
 
+/// Metadata gotten through the parent of a nominal type's metadata.
 class ParentMetadataSource final : public MetadataSource {
   const MetadataSource *Child;
 public:
@@ -337,6 +341,41 @@ public:
   }
 };
 
+/// A source of metadata from the Self metadata parameter passed via
+/// a witness_method convention function.
+class SelfMetadataSource final : public MetadataSource {
+public:
+  SelfMetadataSource() : MetadataSource(MetadataSourceKind::Self) {}
+
+  template <typename Allocator>
+  static const SelfMetadataSource*
+  create(Allocator &A) {
+    return A.template make_source<SelfMetadataSource>();
+  }
+
+  static bool classof(const MetadataSource *MS) {
+    return MS->getKind() == MetadataSourceKind::Self;
+  }
+};
+
+/// A source of metadata from the Self witness table parameter passed via
+/// a witness_method convention function.
+class SelfWitnessTableMetadataSource final : public MetadataSource {
+public:
+  SelfWitnessTableMetadataSource()
+    : MetadataSource(MetadataSourceKind::SelfWitnessTable) {}
+
+  template <typename Allocator>
+  static const SelfWitnessTableMetadataSource*
+  create(Allocator &A) {
+    return A.template make_source<SelfWitnessTableMetadataSource>();
+  }
+
+  static bool classof(const MetadataSource *MS) {
+    return MS->getKind() == MetadataSourceKind::SelfWitnessTable;
+  }
+};
+
 template <typename ImplClass, typename RetTy = void, typename... Args>
 class MetadataSourceVisitor {
 public:

include/swift/Reflection/MetadataSourceBuilder.h

@@ -62,6 +62,16 @@ public:
   createParent(const MetadataSource *Child) {
     return ParentMetadataSource::create(*this, Child);
   }
+
+  const SelfMetadataSource *
+  createSelf() {
+    return SelfMetadataSource::create(*this);
+  }
+
+  const SelfWitnessTableMetadataSource *
+  createSelfWitnessTable() {
+    return SelfWitnessTableMetadataSource::create(*this);
+  }
 };
 
 } // end namespace reflection

include/swift/Reflection/MetadataSources.def

@@ -21,3 +21,5 @@ METADATA_SOURCE(ReferenceCapture, MetadataSource)
 METADATA_SOURCE(MetadataCapture, MetadataSource)
 METADATA_SOURCE(GenericArgument, MetadataSource)
 METADATA_SOURCE(Parent, MetadataSource)
+METADATA_SOURCE(Self, MetadataSource)
+METADATA_SOURCE(SelfWitnessTable, MetadataSource)