Recommit r80858 again (which has been backed out in r80871).

This time I did a self-hosted bootstrap on Linux x86-64,
with no problems. Let's see how darwin 64-bit self-hosting
goes. At the first sign of failure I'll back this out.

Maybe the valgrind bots give me a hint of what may be wrong
(it at all).

llvm-svn: 98957