MetadataSource Improvements: Builder; Parent, Impossible kinds

Create a builder divorced from the ReflectionContext so that
MetadataSources can be created in other contexts, such as emitting
private heap metadata during IRGen, where we'll have to record the
layout of captures and how to get metadata for generic arguments in
order to construct typerefs of the captures, etc.

Add Parent, Metadata capture, and Impossible metadata sources.

stdlib/public/Reflection/MetadataSource.cpp

@@ -12,6 +12,8 @@
 
 #include "swift/Reflection/MetadataSource.h"
 
+#include <sstream>
+
 using namespace swift;
 using namespace reflection;
 
@@ -70,6 +72,13 @@ public:
     closeForm();
   }
 
+  void
+  visitMetadataCaptureMetadataSource(const MetadataCaptureMetadataSource *MC){
+    printHeader("metadata-capture");
+    printField("index", MC->getIndex());
+    closeForm();
+  }
+
   void
   visitGenericArgumentMetadataSource(const GenericArgumentMetadataSource *GA) {
     printHeader("generic-argument");
@@ -77,6 +86,67 @@ public:
     printRec(GA->getSource());
     closeForm();
   }
+
+  void
+  visitParentMetadataSource(const ParentMetadataSource *P) {
+    printHeader("parent-of");
+    printRec(P->getChild());
+    closeForm();
+  }
+
+  void
+  visitImpossibleMetadataSource(const ImpossibleMetadataSource *I) {
+    printHeader("impossible");
+    closeForm();
+  }
+};
+
+class MetadataSourceEncoder
+  : public MetadataSourceVisitor<MetadataSourceEncoder> {
+  std::stringstream OS;
+  bool Finalized  = false;
+public:
+  void
+  visitClosureBindingMetadataSource(const ClosureBindingMetadataSource *CB) {
+    OS << 'B';
+    OS << CB->getIndex();
+  }
+
+  void
+  visitReferenceCaptureMetadataSource(const ReferenceCaptureMetadataSource *RC){
+    OS << 'R';
+    OS << RC->getIndex();
+  }
+
+  void
+  visitMetadataCaptureMetadataSource(const MetadataCaptureMetadataSource *MC) {
+    OS << 'M';
+    OS << MC->getIndex();
+  }
+
+  void
+  visitGenericArgumentMetadataSource(const GenericArgumentMetadataSource *GA) {
+    OS << 'G';
+    OS << GA->getIndex();
+    visit(GA->getSource());
+    OS << '_';
+  }
+
+  void visitParentMetadataSource(const ParentMetadataSource *P) {
+    OS << 'P';
+    visit(P->getChild());
+    OS << '_';
+  }
+
+  void visitImpossibleMetadataSource(const ImpossibleMetadataSource *I) {
+    OS << 'I';
+  }
+
+  std::string finalize() {
+    assert(!Finalized && "Attempted to reuse finalized MetadataSourceEncoder");
+    Finalized = true;
+    return OS.str();
+  }
 };
 
 void MetadataSource::dump() const {
@@ -85,5 +155,15 @@ void MetadataSource::dump() const {
 
 void MetadataSource::dump(std::ostream &OS, unsigned Indent) const {
   PrintMetadataSource(OS, Indent).visit(this);
+  OS << std::endl;
 }
 
+std::string MetadataSource::encode() const {
+  MetadataSourceEncoder Encoder;
+  Encoder.visit(this);
+  return Encoder.finalize();
+}
+
+const ImpossibleMetadataSource *
+ImpossibleMetadataSource::Singleton = new ImpossibleMetadataSource();
+