[clang] [llvm] Singleton hack of fixing static initialisation order fiasco (PR #154541)

[Mircea Trofin via cfe-commits]

mtrofin wrote:

> For FeatureMap, I'm not sure if it's supposed to be a plugin mechanism, or it can just be a member variable MLInlineAdvisor. The comments don't really explain why it's exposed in the header.

@svkeerthy - @efriedma-quic is right, we should probably just make a local copy of the global FeatureMap and mutate that copy with the ir2vec extra features.

https://github.com/llvm/llvm-project/pull/154541