IR: Drop newline from AssemblyWriter::printMDNodeBody()

Remove a newline from `AssemblyWriter::printMDNodeBody()`, and add one
to `AssemblyWriter::writeMDNode()`.  NFCI for assembly output.

However, this drops an inconsistent newline from `Metadata::print()`
when `this` is an `MDNode`.  Now the newline added by `Metadata::dump()`
won't look so verbose.

llvm-svn: 230565
diff --git a/llvm/unittests/IR/MetadataTest.cpp b/llvm/unittests/IR/MetadataTest.cpp
index 849616b..6f2372d 100644
--- a/llvm/unittests/IR/MetadataTest.cpp
+++ b/llvm/unittests/IR/MetadataTest.cpp
@@ -228,7 +228,7 @@
     MDNode *Nodes[] = {N0, N1, N2};
     for (auto *Node : Nodes)
       OS << ", <" << (void *)Node << ">";
-    OS << "}\n";
+    OS << "}";
   }
 
   std::string Actual;