Merge commit f97936fab from llvm git (by Eric Fiselier):
Make page busy state deterministic on free. Pages must be xbusy when removed from objects including calls to free. Pages must not be xbusy when freed and not on an object. Strengthen assertions to match these expectations. In practice very little code had to change busy handling to meet these rules but we can now make stronger guarantees to busy holders and avoid conditionally dropping busy in free.
Add vmgenc(4) driver for ACPI VM generation counter
Vendor import of llvm-project tag llvmorg-9.0.1: https://github.com/llvm/llvm-project/tree/llvmorg-9.0.1
Eliminate differences between Git version of llvm-project release/9.x, and our vendor import in ^/vendor/llvm-project/release-9.x.
Remove empty dirs under vendor/llvm-project, which are no longer stored in git.
Copy llvm vendor/*/dist-release_90 to vendor/llvm-project/release-9.x