Let openmp depend on llvm source, D87841

I’ve posted a technically trivial change that replaces a number of copied lines with an #include from within the llvm tree.

This introduces a dependency edge from the compile time of openmp (currently only the amdgpu runtime part) to the source of llvm. Therefore a long list of reviewers and this email.

Please speak up in favour on the review, or present an opposing argument if necessary. I’d really like to get rid of the copy and paste between the directories within the monorepo, here and elsewhere.

Longer term there’s a bunch of other things we might want to use from LLVM, but short term I just don’t want to copy&rot the various magic numbers.

Thanks all