[clang-format] Rewrite how indent is reduced for compacted namespaces

The previous version set the indentation directly using IndentForLevel,
however, this has a few caveats, namely:

* IndentForLevel applies to all scopes of the entire program being
  formatted, but this indentation should only be adjusted for scopes
  of namespaces.

* The method it used only set the correct indent amount if one wasn't
  already set for a given level, meaning it didn't work correctly if
  anything with indentation preceded a namespace keyword. This
  includes preprocessing directives if using IndentPPDirectives.

This patch instead reduces the Level of all lines within namespaces
which are compacted by CompactNamespaces. This means they will get
correct indentation using the normal process.

Fixes https://github.com/llvm/llvm-project/issues/60843

Reviewed By: owenpan, MyDeveloperDay, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D144296