[clang-tools-extra] [clang-tidy] Fix a crash in bugprone-std-namespace-modification (PR #214704)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

I think the assertion is correct. This PR doesn't fix the root cause.

I think a FIXME comment or a GitHub issue is needed. Otherwise LGTM.

https://github.com/llvm/llvm-project/pull/214704