[RFC] unbundle LLVM?

Not sure how realistic this is, but the compilation of this package is spending _a lot_ of time to recompile llvm/lib, which we're already doing in https://github.com/conda-forge/llvmdev-feedstock. There's other parts coming from MLIR, where I haven't check if we're building that separately somewhere.

Not sure if the surgery for this is feasible TBH, but thought it might be worth discussing. There's much more to (potentially) unbundle, see the upstream [third_party](https://github.com/tensorflow/tensorflow/tree/master/third_party) folder... This also shows another potential problem, because tensorflow vendoring LLVM means that they're not necessarily in sync with (the ABI of) released versions.

CC @xhochy @isuruf @hmaarrfk