C++: Don't strip comments from preprocessed source code.

Change-Id: I3a4817d36b9c724abca504c42914a73f97ab34c7
Reviewed-on: http://codereview.qt.nokia.com/3480
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/libs/cplusplus/FastPreprocessor.cpp

@@ -45,6 +45,7 @@ FastPreprocessor::FastPreprocessor(const Snapshot &snapshot)
 QByteArray FastPreprocessor::run(QString fileName, const QString &source)
 {
     _preproc.setExpandMacros(false);
+    _preproc.setKeepComments(true);
 
     if (Document::Ptr doc = _snapshot.document(fileName)) {
         _merged.insert(fileName);