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

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

Upgrade our copy of llvm/clang to r168974, from upstream's release_32
branch. This is effectively llvm/clang 3.2 RC2; the 3.2 release is
coming soon.

Vendor import of clang release_32 branch r168974 (effectively, 3.2 RC2):
http://llvm.org/svn/llvm-project/cfe/branches/release_32@168974