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

[Dmitry Vasilyev via llvm-commits]

slydiman wrote:

> Ah this was failing in the new test I added. I guess we never previously called `prepend` with an empty `string_view` in our demangling test-suite. Are we sure we want to now support that use-case (as opposed to removing the problematic test)? . If so, we also need to make the change on the libcxxabi side

I think the size checking is useful. I will add it to libcxxabi.

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