[clang] Add -fkeep-system-includes modifier for -E (PR #67684)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This too fails the formatter.  That said, I have no comments, though preprocessing isn't my place of expertise.  Most of the changes are pretty mild though, and the approach of just replacing the output file with a null-OS seems appropriate.  Fix the formatting, and I'll accept, but please give others (Aaron in particular) time to take a look before committing.

https://github.com/llvm/llvm-project/pull/67684