[ADT][NFC] Clang-format DenseMap and DenseSet (#108162)

This is a preparation for upcoming changes to Dense[Map|Set] regarding
hardening against OOM scenarios (see [this
RFC](https://discourse.llvm.org/t/rfc-malfunction-safe-densemap-denseset/81036/7)).
We have changed a lot of code inside Dense[Map|Set] and this preparation
change helps to isolate the relevant parts from pure formatting stuff.

Author: marcauberer