Reland "SafeStack: Delay thread stack clean-up""

This relands commit r339405 (reverted in commit r339408.) The original
revert was due to tests failing on a darwin buildbot; however, after
looking at the affected code more I realized that the Darwin safestack
support has always been broken and disabled it in r339719. This relands
the original commit.

llvm-svn: 339723