Revert the removal of PPCallbacks::PragmaComment() in r201821

The pp-trace clang tool was using it successfully.  We can still delete
the callbacks in Frontend/PrintPreprocessedOutput.cpp because they were
effectively dead.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201825 91177308-0d34-0410-b5e6-96231b3b80d8