No way to allow an unknown lint on the command line

[tmandry]

When migrating a large codebase to a new compiler version, frequently there are new lints. In codebases that enable -Dwarnings, this causes a problem: we'd like to allow the new lint temporarily before fixing in-tree, but passing -Anew-lint-here to the old compiler gives us an error. If we can't upgrade the compiler and build flags in lock step, we are stuck.

-Aunknown-lints exists, but does not have the desired effect on the command line:

$ rustc -Aunknown-lints -Alegacy-derive-helpers hello.rs
error[E0602]: unknown lint: `legacy_derive_helpers`
  |
  = note: requested on the command line with `-A legacy_derive_helpers`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0602`.

Since the same problem that unknown-lints solves exists on the command line, I'd argue that this is a bug in the CLI.

As a corollary, it would be nice if we could do something like -Aunknown-lints -Alegacy-derive-helpers -Dunknown-lints to prevent misspellings elsewhere in the command line.

[jyn514]

Duplicate of #74722.