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

[Paul T Robinson via cfe-commits]

pogo59 wrote:

@jrtc27 I've beefed up the help text some. I'm unable to find any "rendered documentation" for `-E` and friends; if you can point me to it, I'd be happy to add the new option there.

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