Upgrade our copies of clang, llvm, lld, lldb, compiler-rt and libc++ to
6.0.1 release (upstream r335540).

Relnotes: yes
MFC after: 2 weeks

Merge llvm trunk r321017 to contrib/llvm.

Merge llvm trunk r300422 and resolve conflicts.

Update llvm to trunk r290819 and resolve conflicts.