Auto merge of #14232 - HKalbasi:mir, r=Veykril

MIR episode 2

This PR adds:
1. `need-mut` and `unused-mut` diagnostics
2. `View mir` command which shows MIR for the body under cursor, useful for debugging
3. MIR lowering for or-patterns and for-loops

crates/test-utils/src/minicore.rs

@@ -762,6 +762,20 @@ pub mod iter {
                     self
                 }
             }
+            pub struct IntoIter<T, const N: usize>([T; N]);
+            impl<T, const N: usize> IntoIterator for [T; N] {
+                type Item = T;
+                type IntoIter = IntoIter<T, N>;
+                fn into_iter(self) -> I {
+                    IntoIter(self)
+                }
+            }
+            impl<T, const N: usize> Iterator for IntoIter<T, N> {
+                type Item = T;
+                fn next(&mut self) -> Option<T> {
+                    loop {}
+                }
+            }
         }
         pub use self::collect::IntoIterator;
     }