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