Emit error on #![no_std] + #![no_core]

[bjorn3]

cc thepowersgang/mrustc#145.

@rustbot modify labels: +C-enhancement +P-low +requires-nightly +T-compiler

[rustbot]

Error: Label P-low can only be set by Rust team members

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[bjorn3]

@rustbot modify labels: +C-enhancement +requires-nightly +T-compiler

[petrochenkov]

This is some extra logic in the compiler for rejecting a permanently unstable corner case, that already has some reasonable meaning. I'd say wontfix.

[Mark-Simulacrum]

I also lean towards won't fix on this sort of thing.

[oli-obk]

We've generally not tried to fix anything around no_core. Anything we do here will be severely undertested and mostly unused anyway.