[clang-tools-extra] [clang-tidy] Avoid assert failure on non-identifier names in `readability-container-size-empty` (PR #181557)

[via cfe-commits]

zeyi2 wrote:

```
m at n:~/.../projects/llvm-project$ build/bin/clang-tidy -p=build/ -checks='-*,readability-container-size-empty' clang-tools-extra/clang-tidy/ClangTidy.cpp
14 warnings generated.
Suppressed 14 warnings (6 in non-user code, 8 with check filters).
Use -header-filter=.* or leave it as default to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
```

I've verified that running patched `clang-tidy` on this file will no longer crash :)

https://github.com/llvm/llvm-project/pull/181557