[clang] [Clang] Check PP presence when printing stats (PR #131608)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you for the fix!

Second, that we need a summary explaining the problem and solution.

Is this linked to a bug report, if it is, then we should mention that bug report as well.

I would also like to see a test showing a case where we successfully print the preprocessor stats as well, if we don't already have one.

https://github.com/llvm/llvm-project/pull/131608