[llvm] [llvm][utils] Add summary formatter for SmallBitVector (PR #150542)

[Dave Lee via llvm-commits]

kastiglione wrote:

I did some manual testing:

```cpp
  llvm::SmallBitVector v{10};
  v.set(2, 5);
```

then the output before and after was:

```
(llvm::SmallBitVector) v = [0000000000]
...
(llvm::SmallBitVector) v = [0000011100]
```

https://github.com/llvm/llvm-project/pull/150542