PECOFF: Temporarily add a lock to un-break buildbot.

Looks like there's a threading issue in the COFF reader which makes
buildbot unstable. Probability of crash varies depending on the number
of input. If we are linking a big executalbe, LLD almost always crash.

This patch temporarily adds a lock to guard the reader so that LLD
doesn't crash. I'll investigate and fix the issue as soon as possible
because this patch has negative performance impact.

llvm-svn: 230086