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

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

Revive the LLVM and Clang license files, which were removed in my
too-thorough cleanup of unused files, in r213695. Also make sure these
get installed under /usr/share/doc.

Submitted by: rwatson, brooks
Pointy hat to: dim
MFC after: 3 days

Update llvm to r84119.