[dsymutil] Implement the --threads option

This patch adds the --threads option to dsymutil to process
architectures in parallel. The feature is already present in the version
distributed with Xcode, but was not yet upstreamed.

This is NFC as far as the linking behavior is concerned. As threads are
used automatically, the current tests cover the change in
implementation.

Differential revision: https://reviews.llvm.org/D39355

llvm-svn: 316999