[clang] [llvm] [ThinLTO][SplitModuleCG] Add call-graph based module splitting (1/N) (PR #198702)

[Shilei Tian via llvm-commits]

shiltian wrote:

The core of this PR looks very similar to https://github.com/llvm/llvm-project/pull/89245. Some of the code is even identical.

I still think we likely don't need a new pass. Instead, we could build on the existing infrastructure by moving things around in `AMDGPUSplitModule.cpp` and `SplitModule.cpp`, then implement the new split wherever we need it (as well as the two existing passes) using the shared code. I think it is feasible, but indeed needs some real work.

https://github.com/llvm/llvm-project/pull/198702