Preprocessor: initialize member introduced in r173881.

Compilation always sets this explicitly, but creating a preprocessor
manually should still put the 'IsPreprocessedOutput' flag in a valid state.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174077 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Lex/Preprocessor.cpp b/lib/Lex/Preprocessor.cpp
index 4c3c181..eb498db 100644
--- a/lib/Lex/Preprocessor.cpp
+++ b/lib/Lex/Preprocessor.cpp
@@ -97,6 +97,7 @@
   NumCachedTokenLexers = 0;
   PragmasEnabled = true;
   ParsingIfOrElifDirective = false;
+  PreprocessedOutput = false;
 
   CachedLexPos = 0;