[Lex] Allow to consume tokens while preprocessing

Summary:
By adding a hook to consume all tokens produced by the preprocessor.
The intention of this change is to make it possible to consume the
expanded tokens without re-runnig the preprocessor with minimal changes
to the preprocessor and minimal performance penalty when preprocessing
without recording the tokens.

The added hook is very low-level and reconstructing the expanded token
stream requires more work in the client code, the actual algorithm to
collect the tokens using this hook can be found in the follow-up change.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: eraman, nemanjai, kbarton, jsji, riccibruno, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59885

llvm-svn: 361007
diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index b53e6ea..78e0c04 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/clang/lib/Parse/ParseTemplate.cpp
@@ -919,7 +919,7 @@
     PrevTokLocation = RAngleLoc;
   } else {
     PrevTokLocation = TokBeforeGreaterLoc;
-    PP.EnterToken(Tok);
+    PP.EnterToken(Tok, /*IsReinject=*/true);
     Tok = Greater;
   }
 
@@ -1402,7 +1402,7 @@
   // Append the current token at the end of the new token stream so that it
   // doesn't get lost.
   LPT.Toks.push_back(Tok);
-  PP.EnterTokenStream(LPT.Toks, true);
+  PP.EnterTokenStream(LPT.Toks, true, /*IsReinject*/true);
 
   // Consume the previously pushed token.
   ConsumeAnyToken(/*ConsumeCodeCompletionTok=*/true);