Move all sources from the llvm project into contrib/llvm-project.

This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That i

Move all sources from the llvm project into contrib/llvm-project.

This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.

Updating the vendor area to match this layout is next.

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Merge llvm trunk r366426, resolve conflicts, and update FREEBSD-Xlist.

Merge llvm trunk r351319, resolve conflicts, and update FREEBSD-Xlist.

Merge llvm trunk r338150, and resolve conflicts.

Merge llvm trunk r238337 from ^/vendor/llvm/dist, resolve conflicts, and
preserve our customizations, where necessary.