[llvm] [SandboxIR] Use std::make_unique (NFC) (PR #119824)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> The reason we were using `std::uniuqe_ptr` with `new` is because the `CatchSwitchInst` is supposed to be private such that creation of object is always done through the API in `Context`. But I just checked and ithe constructor is public, not private. This PR should fix it: #119901

Thank you for the explanation!  I just LGTMed #119901.

https://github.com/llvm/llvm-project/pull/119824