Add a warning when enabling LTO for crates with both cargo-types that support LTO and ones that don't.

[Lymia]

As discussed in #2301, cargo silently builds a cdylib without LTO when the same crate also has rlib outputs, as rustc would otherwise error.

The only sign this has happened currently is the output binaries being way larger than expected. A warning would be very helpful for this.

[stale[bot]]

As there hasn't been any activity here in over 6 months I've marked this as stale and if no further activity happens for 7 days I will close it.

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[Lymia]

As far as I am aware, this is still an issue.

[weihanglo]

https://github.com/rust-lang/cargo/issues/14612#issue-2554861019 propose to emit a warning suggesting to use cargo rustc --crate-type=X, which is a flag relatively new to this old issue.

I have not yet checked how hard the warning implementation would be. Guess it won't be too hard but a bit gnarly.