[clang] [Clang][NFC] Capture by ref to avoid copying std::string (PR #138231)

[Balazs Benics via cfe-commits]

https://github.com/steakhal approved this pull request.

Makes sense, thanks.
I just wonder why is BName an owning string? To me it would make a lot more sense if it was a StringRef owned by the ASTContext.
WDYT?

https://github.com/llvm/llvm-project/pull/138231