[clang-format] FixNamespaceComments does not understand namespace aliases

https://github.com/llvm/llvm-project/issues/35876

Ensure a namespace alias doesn't get incorrectly identifier as a namespace

Reviewed By: HazardyKnusperkeks, curdeius, owenpan

Differential Revision: https://reviews.llvm.org/D115647

Fixes: #35876