Use magic ExeUnload handle to tag "nothing to be unloaded".

This disambiguates a transaction without JITted module from one with JITted code.
Only the latter should be unloaded.

lib/Interpreter/IncrementalJIT.cpp

@@ -237,6 +237,8 @@ size_t IncrementalJIT::addModules(std::vector<llvm::Module*>&& modules) {
 
 
 void IncrementalJIT::removeModules(size_t handle) {
+  if (handle == (size_t)-1)
+    return;
   m_LazyEmitLayer.removeModuleSet(m_UnloadPoints[handle]);
 }
 

lib/Interpreter/Transaction.cpp

@@ -43,7 +43,7 @@ namespace cling {
     m_IssuedDiags = kNone;
     m_Opts = CompilationOptions();
     m_Module = 0;
-    m_ExeUnload = {0};
+    m_ExeUnload = {(void*)(size_t)-1};
     m_WrapperFD = 0;
     m_Next = 0;
     //m_Sema = S;