Export #[inline] fns with extern indicators

[cofibrant]

In ancient history (#36280) we stopped #[inline] fns being codegened if they weren't used. However,

• Warn if a function is marked as both #[inline] and #[no_mangle] #72944
• #[inline] prevents symbol from appearing in staticlib output #72463

observe that when writing something like

#![crate_type = "cdylib"]

#[no_mangle]
#[inline]
pub extern "C" fn foo() {
    // ...
}

we really do want foo to be codegened. This change makes this the case.

Resolves #72944, resolves #72463 (and maybe some more)