[llvm-cov] Don't spawn a thread unless ThreadCount > 1

Initializing a ThreadPool with ThreadCount = 1 spawns a thread even
though we don't need to. This is at least slower than it needs to be,
and at worst may somehow be exacerbating PR30735 (llvm-cov times out
on ARM bots).

As a follow-up, I'll try to add logic to llvm::ThreadPool to avoid
spawning a thread when ThreadCount = 1.

llvm-svn: 284621