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

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

I mean places like https://clang.llvm.org/docs/ClangCommandLineReference.html and https://clang.llvm.org/docs/UsersManual.html#command-line-options.

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