Treat RETURN_PLACE as a normal Local

Copy its value to the `return_place` upon leaving a call frame

src/test/ui/consts/const-eval/ub-nonnull.stderr

@@ -13,7 +13,7 @@ LL | / const OUT_OF_BOUNDS_PTR: NonNull<u8> = { unsafe {
 LL | |     let ptr: &[u8; 256] = mem::transmute(&0u8); // &0 gets promoted so it does not dangle
 LL | |     // Use address-of-element for pointer arithmetic. This could wrap around to NULL!
 LL | |     let out_of_bounds_ptr = &ptr[255];
-   | |                             ^^^^^^^^^ Memory access failed: pointer must be in-bounds at offset 256, but is outside bounds of alloc8 which has size 1
+   | |                             ^^^^^^^^^ Memory access failed: pointer must be in-bounds at offset 256, but is outside bounds of alloc11 which has size 1
 LL | |     mem::transmute(out_of_bounds_ptr)
 LL | | } };
    | |____-

src/test/ui/consts/miri_unleashed/mutable_const.stderr

@@ -11,7 +11,7 @@ LL | / const MUTATING_BEHIND_RAW: () = {
 LL | |     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
 LL | |     unsafe {
 LL | |         *MUTABLE_BEHIND_RAW = 99
-   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ writing to alloc1 which is read-only
+   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ writing to alloc2 which is read-only
 LL | |     }
 LL | | };
    | |__-