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

[Rahul Joshi via llvm-commits]

jurahul wrote:

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

Agreed, demoting this to a draft for now.

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