Implement reflection support for Symbolic Extended Existential types.

This patch adds a new SymbolicExtendedExistentialTypeRef kind, and
wires it up in TypeDecoder, TypeRefBuilder, TypeLowering, and
ASTDemangler.

This is tested indirectly via the matching LLDB commit.

include/swift/Demangling/Demangle.h

@@ -193,6 +193,7 @@ public:
   };
 
   using IndexType = uint64_t;
+  using RemoteAddressType = std::pair<uint64_t, uint8_t>;
 
   friend class NodeFactory;
   
@@ -209,14 +210,20 @@ private:
     IndexType Index;
     NodePointer InlineChildren[2];
     NodeVector Children;
+    RemoteAddressType RemoteAddress;
   };
 
 
   Kind NodeKind;
 
   enum class PayloadKind : uint8_t {
-    None = 0, OneChild = 1, TwoChildren = 2,
-    Text, Index, ManyChildren
+    None = 0,
+    OneChild = 1,
+    TwoChildren = 2,
+    Text,
+    Index,
+    ManyChildren,
+    RemoteAddress
   };
   PayloadKind NodePayloadKind;
 
@@ -231,6 +238,10 @@ private:
       : NodeKind(k), NodePayloadKind(PayloadKind::Index) {
     Index = index;
   }
+  Node(Kind k, uint64_t remoteAddress, uint8_t addressSpace)
+      : NodeKind(k), NodePayloadKind(PayloadKind::RemoteAddress) {
+    RemoteAddress = {remoteAddress, addressSpace};
+  }
   Node(const Node &) = delete;
   Node &operator=(const Node &) = delete;
 
@@ -284,6 +295,14 @@ public:
     return Index;
   }
 
+  bool hasRemoteAddress() const {
+    return NodePayloadKind == PayloadKind::RemoteAddress;
+  }
+  std::pair<uint64_t, uint8_t> getRemoteAddress() const {
+    assert(hasRemoteAddress());
+    return RemoteAddress;
+  }
+
   using iterator = const NodePointer *;
 
   size_t getNumChildren() const {