[llvm-cov] Prevent llvm-cov from using too many threads

As reported here: https://reviews.llvm.org/D75153#1987272

Before, each instance of llvm-cov was creating one thread per hardware core, which wasn't needed probably because the number of inputs were small. This was probably causing a thread rlimit issue on large core count systems.

After this patch, the previous behavior is restored (to what was before rG8404aeb5):

If --num-threads is not specified, we create one thread per input, up to num.cores.
When specified, --num-threads indicates any number of threads, with no upper limit.

Differential Revision: https://reviews.llvm.org/D78408