[llvm-branch-commits] [llvm, BOLT, polly] Remove unused DenseMapInfo::getTombstoneKey (PR #200628)

[Fangrui Song via llvm-branch-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/200628

#200595 changed DenseMap to no longer create tombstone buckets, so
DenseMapInfo<T>::getTombstoneKey() is never called. Remove dead
definitions and dead tombstone branches.