[llvm] [offload][SYCL] Add SYCL Module splitting. (PR #131347)

[Maksim Sabianin via llvm-commits]

maksimsab wrote:

@jhuber6 @jdoerfert 

Could you please share your opinions on the latest changes?

I am a bit skeptical about recent `FunctionCategorizer` design since other OffloadKind users would rather interested of reusing a `thin-LTO` framework. The current algorithm is straightforward and non-parallel. Once we start migrating it to parallel version I see it is going to have 2 stages:
1) Prepare a plan of moving functions/objects from input Modules to output parts.
2) Execute the plan parallelly according to the plan.

We have been discussing the first step in this PR. I think it could be ironed separately in an effort dedicated to `thin-LTO` migration.

https://github.com/llvm/llvm-project/pull/131347