./x.py: add --only lint1,lint2 flag to ./x.py clippy which only runs specified lints and ignores all others

[matthiaskrgr]

for example:
./x.py clippy --only or_fun_call,type_complexity
will only run these two clippy lints.

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

Hm, I guess this doesn't seem bad but feels like rustbuild is really the wrong place for it... I am wondering what you think about trying to propose this for clippy, at which point something like CLIPPYFLAGS="--only foo" x.py clippy would work. Presumably it's a more widespread need than just the rustc codebase?

If that feels wrong though this approach seems fine, not too complicated.

[matthiaskrgr]

In the clippy readme, we have already documented that one can run cargo clippy -- -Aclippy::all -Wclippy::only_this_one for a single project, but for x.py, simply putting this into RUSTFLAGS does not work unfortunately which is why I added the workaround

 error[E0602]: unknown lint: `clippy::all`
  |
  = note: requested on the command line with `-A clippy::all`

[Mark-Simulacrum]

Hm, I would like to fix this by fixing that - I'm not sure why it's not working, but seems plausible that it's because there's a non-clippy cargo running to build bootstrap itself. Could we do this with a general clippy args variable (or just x.py clippy -- <args here>)

[Mark-Simulacrum]

@matthiaskrgr I think I recently saw a comment to the effect of you having done this somewhere else, but maybe that was in this PR?

I am leaning towards closing this PR unless it switches to the "general arguments" shape i.e., x.py clippy -- --any-args-here.