Rollup merge of rust-lang#128170 - saethlin:clone-fn, r=compiler-errors

Make Clone::clone a lang item

I want to absorb all the logic for picking whether an Instance is LocalCopy or GloballyShared into one place. As part of this, I wanted to identify Clone shims inside `cross_crate_inlinable` and found that rather tricky. `@compiler-errors` suggested that I add a lang item for `Clone::clone` because that would produce other cleanups in the compiler.

That sounds good to me, but I have looked and I've only been able to find one.

r? compiler-errors

Author: tgross35

core/src/clone.rs

@@ -160,6 +160,9 @@ pub trait Clone: Sized {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "cloning is often expensive and is not expected to have side effects"]
+    // Clone::clone is special because the compiler generates MIR to implement it for some types.
+    // See InstanceKind::CloneShim.
+    #[cfg_attr(not(bootstrap), lang = "clone_fn")]
     fn clone(&self) -> Self;
 
     /// Performs copy-assignment from `source`.