[clang] [clang-tools-extra] [clang][clang-tools-extra] Remove unused DenseMapInfo::getEmptyKey (PR #201987)

[Alexis Engelke via cfe-commits]

https://github.com/aengelke approved this pull request.

LGTM. Clang-format fixes should, if you want them fixed, be a separate commit.

https://github.com/llvm/llvm-project/pull/201987