first step refactoring driver so that it can process multiple input files
at once.

llvm-svn: 39207
diff --git a/clang/Driver/PrintPreprocessedOutput.cpp b/clang/Driver/PrintPreprocessedOutput.cpp
index 931404f..fb5208d 100644
--- a/clang/Driver/PrintPreprocessedOutput.cpp
+++ b/clang/Driver/PrintPreprocessedOutput.cpp
@@ -387,6 +387,7 @@
 ///
 void clang::DoPrintPreprocessedInput(unsigned MainFileID, Preprocessor &PP,
                                      LangOptions &Options) {
+  // FIXME: should not mutate LangOptions!!
   if (EnableCommentOutput)          // -C specified?
     Options.KeepComments = 1;
   if (EnableMacroCommentOutput)     // -CC specified?