[llvm-cov] Use a thread pool to speed up report generation (NFC)

It's safe to print out source coverage views using multiple threads when
using the -output-dir mode of the `llvm-cov show` sub-command.

While testing this on my development machine, I observed that the speed
up is roughly linear with the number of available cores. Avg. time for
`llvm-cov show ./llvm-as -show-line-counts-or-regions`:

    1 thread: 7.79s user 0.33s system 98% cpu 8.228 total
    4 threads: 7.82s user 0.34s system 283% cpu 2.880 total

llvm-svn: 275321