Merge ^/vendor/llvm/dist up to its last change, and resolve conflicts.

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, clang, lld, lldb, compiler-rt and libc++ r306956, and update
build glue.

Merge llvm trunk r300422 and resolve conflicts.