Add the MDBuilder helper class for conveniently creating metadata.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154766 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.html

@@ -532,6 +532,8 @@ syntax, there are still significant gaps in that support.</p>
 <li><code>llvm::getTrapFunctionName()</code></li>
 <li><code>llvm::EnableSegmentedStacks</code></li>
 </ul></li>
+  <li>The MDBuilder class has been added to simplify the creation of
+      metadata.</li>
   <li>....</li>
 </ul>