[llvm] [llvm] Fixed ItaniumDemangle.h compiled by MSVC (PR #137409)

[Michael Buch via llvm-commits]

Michael137 wrote:

I already fixed this in a separate commit by just not using the `friend class` idiom: https://github.com/llvm/llvm-project/commit/bd96fa778809d70e688d27cae2f7667b2aed69c0

https://github.com/llvm/llvm-project/pull/137409