Enabling internal lints shouldn't be dependent on `-Zunstable-options`

[compiler-errors]

We currently enable internal lints only if -Zunstable-options is passed:

rust/compiler/rustc_session/src/session.rs

Lines 344 to 349 in 1447f9d

	/// Returns `true` if internal lints should be added to the lint store - i.e. if
	/// `-Zunstable-options` is provided and this isn't rustdoc (internal lints can trigger errors
	/// to be emitted under rustdoc).
	pub fn enable_internal_lints(&self) -> bool {
	self.unstable_options() && !self.opts.actually_rustdoc
	}

This behavior is surprising at least, and seems wrong. Should we move this to some -Zenable-internal-lints or something?

[compiler-errors]

@matthiaskrgr this is why lmfao

[matthiaskrgr]

fwiw clippy solves this by having an internal lint group which is only compiled-in for ci-tests