Revert "uninit intrinsic is gone"

This reverts commit fa290f1.
Uninit is being reinstated because it breaks some broken code.

src/shims/intrinsics.rs

@@ -522,6 +522,36 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_scalar(res, dest)?;
             }
 
+            "uninit" => {
+                // Check fast path: we don't want to force an allocation in case the destination is a simple value,
+                // but we also do not want to create a new allocation with 0s and then copy that over.
+                // FIXME: We do not properly validate in case of ZSTs and when doing it in memory!
+                // However, this only affects direct calls of the intrinsic; calls to the stable
+                // functions wrapping them do get their validation.
+                // FIXME: should we check alignment for ZSTs?
+                if !dest.layout.is_zst() {
+                    match dest.layout.abi {
+                        layout::Abi::Scalar(..) => {
+                            let x = ScalarMaybeUndef::Undef;
+                            this.write_immediate(Immediate::Scalar(x), dest)?;
+                        }
+                        layout::Abi::ScalarPair(..) => {
+                            let x = ScalarMaybeUndef::Undef;
+                            this.write_immediate(Immediate::ScalarPair(x, x), dest)?;
+                        }
+                        _ => {
+                            // Do it in memory
+                            let mplace = this.force_allocation(dest)?;
+                            assert!(mplace.meta.is_none());
+                            let ptr = mplace.ptr.to_ptr()?;
+                            this.memory_mut()
+                                .get_mut(ptr.alloc_id)?
+                                .mark_definedness(ptr, dest.layout.size, false);
+                        }
+                    }
+                }
+            }
+
             "write_bytes" => {
                 let ty = substs.type_at(0);
                 let ty_layout = this.layout_of(ty)?;