The #if now has a conditional expression, so a user can add
`-D__CLANG_REWRITTEN_SYSTEM_INCLUDES` to include the system headers
instead of using the expanded content, or
`-D__CLANG_REWRITTEN_INCLUDES` to include all headers.
Also added the filename to the comments it emits, to help identify where
included text ends, making it easier to identify and remove the content of
individual headers.
4a16b51f43