[llvm] [llvm] Fixed Demangle OutputBuffer (PR #138564)

[Richard Smith via llvm-commits]

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

Looks fine. I guess the problem was that we were passing `memcpy` the pointer `&*R.begin()`, which might be null if the `string_view` is empty?

https://github.com/llvm/llvm-project/pull/138564