[analyzer] Add stop-gap patch to prevent assertion failure when analyzing LLVM codebase.

This potentially reduces a performance optimization of throwing away
PreStmtPurgeDeadSymbols nodes.  I'll investigate the performance impact
soon and see if we need something better.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176149 91177308-0d34-0410-b5e6-96231b3b80d8

lib/StaticAnalyzer/Core/ExplodedGraph.cpp

@@ -110,7 +110,7 @@ bool ExplodedGraph::shouldCollect(const ExplodedNode *node) {
   // analysis history and are not consulted by any client code.
   ProgramPoint progPoint = node->getLocation();
   if (progPoint.getAs<PreStmtPurgeDeadSymbols>())
-    return true;
+    return !progPoint.getTag();
 
   // Condition 3.
   if (!progPoint.getAs<PostStmt>() || progPoint.getAs<PostStore>())