[clang] [clang] Use llvm::count (NFC) (PR #143228)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

You should always have a summary, for this one it could have simply been "replace std::count with llvm::count in Sema::CheckBaseSpecifier"

https://github.com/llvm/llvm-project/pull/143228