Revert r314616: Run writeTo() concurrently.
Revert r314616: Run writeTo() concurrently.

This reverts commit r314616 because nested parallel-for loops can hang.
Our ThreadPoolExecutor is not necessarily able to handle nested calls.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@315098 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld