rustdoc should not require a file to display -Whelp

[matthiaskrgr]

$ rustc -Whelp
<lint list>

$ rustdoc -Whelp
error: missing file operand

You have to do something like rustdoc file.rs -Whelp in order to see the lint list which is a bit weird 😄 (it does not seem to actually process the file then at least...)

[GuillaumeGomez]

Weirdly enough, it seems like we need to have a file to be able to run register_plugins in lib.rs::main_options. If we don't provide a file, the compiler doesn't allow us to build plugins. I think the reason behind this is that rustdoc should always be run with cargo which will always provide a file.

I'm not sure if it's worth fixing but just in case: do you think there is a way to fix this @rust-lang/compiler ?

[jyn514]

Duplicate of #88756 (which has an open PR if anyone wants to help take it over)