[SPARK-20972][SQL] rename HintInfo.isBroadcastable to broadcast

## What changes were proposed in this pull request?

`HintInfo.isBroadcastable` is actually not an accurate name, it's used to force the planner to broadcast a plan no matter what the data size is, via the hint mechanism. I think `forceBroadcast` is a better name.

And `isBroadcastable` only have 2 possible values: `Some(true)` and `None`, so we can just use boolean type for it.

## How was this patch tested?

existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18189 from cloud-fan/stats.