Log to the right stream in DwarfTransformer::handleDie().

Since we might end up using multiple threads when logging information in the DWARFTransformer, the handleDie() method must use the supplied stream named "OS" when logging warnings and errors. When we use multiple threads, we log to a thread specific stream buffer and then use a mutex to ensure our output doesn't overlap when we emit warnings and errors after a thread is done.

Differential Revision: https://reviews.llvm.org/D109401