[clang] [CIR] Record the source level identity of functions on cir.func (PR #207261)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

I'd like to see a mini-design showing the information you intend to collect and what it's going to be used for. That will be useful in reviewing the preliminary aspects of the implementation.

I suspect that we're going to have a problem with either compile time or memory usage. If you attach the information to the functions, as you are doing here, we'll have to do a lookup based on a flat-symbol-ref to find it when performing analysis of the call site. On the other hand, if you attached the information to the call site, the IR would become very bloated very quickly. I think attaching the information to the function is the right way to go, but it's something to keep in mind as we implement this.

https://github.com/llvm/llvm-project/pull/207261