[llvm] [LLVM] Change `Intrinsic::getBaseName` to return `std::string` (PR #111613)

[Nicolai Hähnle via llvm-commits]

nhaehnle wrote:

If we go down this route, we should definitely clean up the users of this function. Many of them want to allocate a `std::string` anyway, but many don't and get be served by getDeclaration or something.

https://github.com/llvm/llvm-project/pull/111613