Fix cling teardown issues when a pch is attached. Kudos to Axel.

lib/Interpreter/Interpreter.cpp

@@ -233,6 +233,11 @@ namespace cling {
     for (size_t i = 0, e = m_StoredStates.size(); i != e; ++i)
       delete m_StoredStates[i];
     getCI()->getDiagnostics().getClient()->EndSourceFile();
+    // We want to keep the callback alive during the shutdown of Sema, CodeGen
+    // and the ASTContext. For that to happen we shut down the IncrementalParser
+    // explicitly, before the implicit destruction (through the unique_ptr) of
+    // the callbacks.
+    m_IncrParser.reset(0);
   }
 
   const char* Interpreter::getVersion() const {