COFF: Parallelize Writer::writeSections().
COFF: Parallelize Writer::writeSections().

Self-hosting took 801 ms on my machine. Of which this function took
69 ms. Now it takes 37 ms. That is about 4% overall performance
improvement.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@248052 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld