[dsymutil] Re-enable threading

Threading was disabled in r317263 because it broke a test in combination
with `-DLLVM_ENABLE_THREADS=OFF`. This was because a ThreadPool warning
was piped to llvm-dwarfdump which was expecting to read an object from
stdin.

This patch re-enables threading and fixes the offending test.
Unfortunately this required more than just moving the ThreadPool out of
the for loop because of the TempFile refactoring that took place in the
meantime.

Differential revision: https://reviews.llvm.org/D41180

llvm-svn: 320601