`fn` nested in `const fn` causes "Cannot create local mono-item for ..." ICE cross-crate.

[eddyb]

// a.rs
#![feature(const_fn)]

pub const fn foo() -> fn() {
    fn bar() {}
    bar
}

// b.rs
extern crate a;
fn main() {
    a::foo()();
}

error: internal compiler error: librustc_mir/monomorphize/collector.rs:757:
Cannot create local mono-item for DefId(10/0:4 ~ a[8787]::foo[0]::bar[0])

Adding #[inline] to bar's definition works around the ICE.
Also, the bug doesn't trigger if foo is an #[inline] non-const fn, so it's not just that foo is inlined cross-crate, but const fn-specific behavior is also involved.

cc @oli-obk @michaelwoerister

[eddyb]

Still happens on the latest nightly. cc @rust-lang/compiler
It doesn't block #49219, but the workaround in that PR is a bit ugly.

[oli-obk]

I have a fix in #56024