Merge lld trunk r321017 to contrib/llvm/tools/lld.

Upgrade our copies of clang, llvm, lld and lldb to r309439 from the
upstream release_50 branch. This is just after upstream's 5.0.0-rc1.

MFC after: 2 months
X-MFC-with: r321369

Update lld to trunk r290819 and resolve conflicts.

Bring lld (release_39 branch, r279477) to contrib