[llvm] [BPF] Implement `canLowerReturn` for big returns (PR #174478)

[Alessandro Decina via llvm-commits]

alessandrod wrote:

> This PR would allow rust function to return u128

To expand a bit on this: the eBPF target for rust is functional, but it's nightly only. One of the requirements for it to be promoted as stable is that the whole rust `core` library must compile. `u128` is a primitive core type, therefore it's a goal to have it work. 

https://github.com/llvm/llvm-project/pull/174478