[clangd] Tracing: name worker threads, and enforce naming scheduled async tasks

Summary:
This has a bit of a blast radius, but I think there's enough value in "forcing"
us to give names to these async tasks for debugging. Guessing about what
multithreaded code is doing is so unfun...

The "file" param attached to the tasks may seem to be redundant with the thread
names, but note that thread names are truncated to 15 chars on linux!
We'll be lucky to get the whole basename...

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D43388

llvm-svn: 325480