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

[Paul T Robinson via cfe-commits]

pogo59 wrote:

> The first link does, somewhat, and seems like the right place to add this new option. I'll have a go at that.

Well, that one appears to be generated from the Options.td file, and simply includes the help text, so in effect I've already done that. :)

UsersManual.html doesn't have a preprocessor options section at all, and I think it's outside the scope of this patch to add one.

clang.html (actually CommandGuide/clang.rst) doesn't really talk about the other `-E` affecting options, but at least it has a subsection about preprocessor options, so I'll see what I can do there.

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