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

[Paul T Robinson via llvm-commits]

pogo59 wrote:

Something got screwed up when I tried to squash all but the first commit together. I put it back together manually. The NFC commit was pushed as 9500616, the functional stuff squashed and committed as 71d83bb.

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