[analyzer] Rename addTrackNullOrUndefValueVisitor to trackNullOrUndefValue.

This helper function (in the clang::ento::bugreporter namespace) may add more
than one visitor, but conceptually it's tracking a single use of a null or
undefined value and should do so as best it can.

Also, the BugReport parameter has been made a reference to underscore that
it is non-optional.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162720 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp b/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp
index 14a884e..8ae75ea 100644
--- a/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp
+++ b/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp
@@ -78,7 +78,7 @@
   BugReport *R = new BugReport(*BT, str, N);
   if (ex) {
     R->addRange(ex->getSourceRange());
-    bugreporter::addTrackNullOrUndefValueVisitor(N, ex, R);
+    bugreporter::trackNullOrUndefValue(N, ex, *R);
   }
   R->disablePathPruning();
   C.EmitReport(R);