Reflection: Use _ instead of - as word separator in metadata source s-expression output

For consistency with TypeRefs, and AST dumping.

Most Lisps use -, but we're C++ programmers here.

Author: slavapestov

stdlib/public/Reflection/MetadataSource.cpp

@@ -60,36 +60,36 @@ class PrintMetadataSource
 
   void
   visitClosureBindingMetadataSource(const ClosureBindingMetadataSource *CB) {
-    printHeader("closure-binding");
+    printHeader("closure_binding");
     printField("index", CB->getIndex());
     closeForm();
   }
 
   void
   visitReferenceCaptureMetadataSource(const ReferenceCaptureMetadataSource *RC){
-    printHeader("reference-capture");
+    printHeader("reference_capture");
     printField("index", RC->getIndex());
     closeForm();
   }
 
   void
   visitMetadataCaptureMetadataSource(const MetadataCaptureMetadataSource *MC){
-    printHeader("metadata-capture");
+    printHeader("metadata_capture");
     printField("index", MC->getIndex());
     closeForm();
   }
 
   void
   visitGenericArgumentMetadataSource(const GenericArgumentMetadataSource *GA) {
-    printHeader("generic-argument");
+    printHeader("generic_argument");
     printField("index", GA->getIndex());
     printRec(GA->getSource());
     closeForm();
   }
 
   void
   visitParentMetadataSource(const ParentMetadataSource *P) {
-    printHeader("parent-of");
+    printHeader("parent_of");
     printRec(P->getChild());
     closeForm();
   }
@@ -101,7 +101,7 @@ class PrintMetadataSource
 
   void
   visitSelfWitnessTableMetadataSource(const SelfWitnessTableMetadataSource *W) {
-    printHeader("self-witness-table");
+    printHeader("self_witness_table");
     closeForm();
   }
 };