[llvm][utils] Add summary formatter for SmallBitVector

[kastiglione]

Originally implemented in swiftlang/swift#29014.

I've made a couple changes:

1. Use the target's address size, not lldb
2. Replaced the loop with a format string

[jimingham]

I don't know enough about how SmallBItVector works to know if this is right or not.

[kastiglione]

I did some manual testing:

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

then the output before and after was:

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

[jimingham]

This can't hurt anything, and if it works in sniff testing is a good sign it's right.
At some point we need to come up with a way of testing the llvm & lldb data formatters but that's obviously a bigger change that's out of scope for this PR.