Previously, when borrowck failed it would taint all promoteds within the MIR
body. An attempt to evaluated the promoteds would subsequently fail with
spurious "note: erroneous constant used". For example:
```console
...
note: erroneous constant used
--> tests/ui/borrowck/tainted-promoteds.rs:7:9
|
7 | a = &0 * &1 * &2 * &3;
| ^^
note: erroneous constant used
--> tests/ui/borrowck/tainted-promoteds.rs:7:14
|
7 | a = &0 * &1 * &2 * &3;
| ^^
note: erroneous constant used
--> tests/ui/borrowck/tainted-promoteds.rs:7:19
|
7 | a = &0 * &1 * &2 * &3;
| ^^
note: erroneous constant used
--> tests/ui/borrowck/tainted-promoteds.rs:7:24
|
7 | a = &0 * &1 * &2 * &3;
| ^^
```
Borrowck failure doesn't indicate that there is anything wrong with
promoteds. Leave them untainted.
