This CL modifies the code so that we can do sequential and parallel
parsing of function blocks in bitcode files, based on a command line
argument.

The command line argument was added because during testing, I had one
compilation failure (transient), and do not know the cause. Hence, I
was reluctant to install this CL without a command-line flag.

To test the new parallel parser, the easiest solution is to edit
IceClFlags.def and set the default value of ParseParallel to true.

This code also fixes up unit parsing tests, as well as one parsing
test.  The cause of these problems was the implicit assumption that
function blocks are parsed sequentially, which no longer applies when
function blocks are parsed in parallel. To fix this, the "threads=0"
command line argument was added. It also added the starting up of
worker threads, since parsing of function blocks will happen in the
translation thread if parallel parsing is turned on.

The OptQ queue was modified to contain OptWorkerItem instances with a
single virtual to get the parsed code. This allows the IceConverter to
continue to work, by simply passing the generated Cfg as a work item.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4363
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1834473002 .