[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/Lex/MacroArgs.cpp b/clang/lib/Lex/MacroArgs.cpp
index 06e3add..5aa4679 100644
--- a/clang/lib/Lex/MacroArgs.cpp
+++ b/clang/lib/Lex/MacroArgs.cpp
@@ -181,7 +181,7 @@
   // list.  With this installed, we lex expanded tokens until we hit the EOF
   // token at the end of the unexp list.
   PP.EnterTokenStream(AT, NumToks, false /*disable expand*/,
-                      false /*owns tokens*/);
+                      false /*owns tokens*/, false /*is reinject*/);
 
   // Lex all of the macro-expanded tokens into Result.
   do {