Allow invoking all help options at once
Implements rust-lang/rust#150442
Help messages are printed in the order they are invoked, but only once (e.g. `--help -C help --help` prints regular help then codegen help).
Lint groups (`-Whelp`) are always printed last due to necessarily coming later in the compiler pipeline.
Adds `help` flags to `-C` and `-Z` to avoid an error in `build_session_options`.
cc @bjorn3 [#t-compiler/major changes > Allow combining `--help -C help -Z help -… compiler-team#954 @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/233931-t-compiler.2Fmajor-changes/topic/Allow.20combining.20.60--help.20-C.20help.20-Z.20help.20-.E2.80.A6.20compiler-team.23954/near/564358190) - this currently maintains the behaviour of unrecognized options always failing-fast, as this happens within `getopt`s parsing, before we can even check if help options are present.
05d494faaf