[lldb] Parallelize fetching symbol files in crashlog.py

When using dsymForUUID, the majority of time symbolication a crashlog
with crashlog.py is spent waiting for it to complete. Currently, we're
calling dsymForUUID sequentially when iterating over the modules. We can
drastically cut down this time by calling dsymForUUID in parallel. This
patch uses Python's ThreadPoolExecutor (introduced in Python 3.2) to
parallelize this IO-bound operation.

The performance improvement is hard to benchmark, because even with an
empty local cache, consecutive calls to dsymForUUID for the same UUID
complete faster. With warm caches, I'm seeing a ~30% performance
improvement (~90s -> ~60s). I suspect the gains will be much bigger for
a cold cache.

dsymForUUID supports batching up multiple UUIDs. I considered going that
route, but that would require more intrusive changes. It would require
hoisting the logic out of locate_module_and_debug_symbols which we
explicitly document [1] as a feature of Symbolication.py to locate
symbol files.

[1] https://lldb.llvm.org/use/symbolication.html

Differential reviison: https://reviews.llvm.org/D125107