Do not reset TUScope when we are in incremental processing mode.

[v.g.vassilev]

Summary

We are using this in the context of cling, where we keep the compiler instance alive after EOF.

The incremental processing part in clang was done mostly by me and this patch LGTM, but I'd like to go under a formal review procedure.

[v.g.vassilev]

Landed in r284372.

lib/Sema/Sema.cpp

 
   if (TUKind == TU_Prefix) {
     // Translation unit prefixes don't need any of the checking below.
-    TUScope = nullptr;
+    if (!PP.isIncrementalProcessingEnabled())
+      TUScope = nullptr;
     return;
   }
 

   assert(ParsingInitForAutoVars.empty() &&
          "Didn't unmark var as having its initializer parsed");
 
-  TUScope = nullptr;
+  if (!PP.isIncrementalProcessingEnabled())
+    TUScope = nullptr;
 }