Bring back the rest of r315721.

The original patch was reported to break a libcxx bot. Unfortunately I
could not reproduce the issue and the bot log is not available anymore.

This patch changes the handling of shared symbols. I will watch the
bot and see it breaks.

Original message:
--------------------------------------------------------------
Handle shared and lazy symbol in the gnu hash construction.

We were not considering those symbols undefined and they could end up
in the end of the dynamic symbol table.
--------------------------------------------------------------

llvm-svn: 316051