[4.0.0 Release] Current status

According to the schedule on llvm.org, we should have tagged 'final'
yesterday, so the release is now officially a little behind schedule.

There are currently seven blockers (llvm.org/pr31622). Several of them
have patches. If you can help reviewing them, please do so.

- PR30256: Assert in llvm::ReassociatePass::OptimizeAdd
  There's a patch in progress that looks promising.

- PR30284: Assertion `!B->isCollapsed() && "Cannot merge from collapsed
  There's a patch (D30242) but it needs more attention.

- PR31181: wrong code with "opt [..] -correlated-propagation [..] -indvars"
  The problem seems somewhat understood but there is no patch.

- PR31729: GVNHoist illegally hoists div instruction
  There is a patch (D29092) that looks promising but currently stuck(?).

- PR31847: After r288115: Assertion failed: (isLoopInvariant(Operands[i], L)
  There is a patch (D29641) but no recent progress.

- PR31863: Clang can't read back a PCH just produced
  There is a patch (D29753) but it's stuck.

- PR31864: incorrect _GCC_ATOMIC_LLONG_LOCK_FREE values on i386-freebsd
  There is a patch (D29542) but it lacks active review.

There are also a few patches in my merge queue waiting for approvals.