[ADT] Clean up FoldingSet (NFC) (#216944)
- Remove the word "simple" from comments referring to "simple Node",
which is a 20-year-old leftover from SelectionDAGCSEMap.
- AllocateBuckets, based on safe_calloc, never returns nullptr.
- In FoldingSetBase::GetOrInsertNode, the parameter type
FoldingSetBase::Node * is simplified to Node * because we are
already in the scope of FoldingSetBase.
Other changes should be self-explanatory.
