Allow invoking all help options at once

[clubby789]

Implements #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 @ 💬 - this currently maintains the behaviour of unrecognized options always failing-fast, as this happens within getopts parsing, before we can even check if help options are present.

[rustbot]

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[jyn514]

could you add some tests for this, please?

[clubby789]

Added tests for the new combined help behavior, as well as testing the current behavior of --help --invalid-flag