CoreSubst: Stricten `substBndr` and `cloneBndr`

Doing so reduced allocations of `cloneBndr` by about 25%.

```
T9233(normal) ghc/alloc    672,488,656    663,083,216  -1.4% GOOD
T9675(optasm) ghc/alloc    423,029,256    415,812,200  -1.7%

    geo. mean                                          -0.1%
    minimum                                            -1.7%
    maximum                                            +0.1%
```

Metric Decrease:
    T9233