Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import LLVM/clang from vendor stripped of docs/ test/ website/ www/ examples/
in llvm/ and/or llvm/contrib/clang/ respectively.

Approved by: ed (mentor)
Approved by: core

Update LLVM to r103004.

Update LLVM to r100520.

Update llvm to r84119.