[ADT] Clean up FoldingSet.* (NFC) (#216619)
This patch cleans up minor issues in FoldingSet.*.
- FoldingSetBase::NodeID never existed in the history of FoldingSet.
The comment should refer to FoldingSetNodeID instead.
- GetBucketPtr lost the first line of its doc comment on February 4,
2008 in commit e2887863563f, leaving only "testing.".
- ImmutableSetTest.cpp was relying on FoldingSet.h to include
ArrayRef.h.
Other changes should be self-explanatory.
Assisted-by: Antigravity
