[llvm-cov] Add a cl::opt to control the number of threads

[llvm-cov] Add a cl::opt to control the number of threads

When an output directory is specified, llvm-cov spawns some threads to
speed up the process of writing out file reports. Add an option which
allows users to control how many threads llvm-cov uses.

A CommandGuide.rst update + test is included.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307609 91177308-0d34-0410-b5e6-96231b3b80d8