Tidy statistics collection

This removes some statistics counters and timers which were not used,
adds new counters and timers for some language features that were not
monitored previously and separates the counters and timers into those
which are of interest for investigating user code and those which are
only of interest to the developer of the runtime itself.
The runtime developer statistics are now ony collected if the
additional #define KMP_DEVELOPER_STATS is set.

Additional user statistics which are now collected include:
* Count of nested parallelism (omp parallel inside a parallel region)
* Count of omp distribute occurrences
* Count of omp teams occurrences
* Counts of task related statistics (taskyield, task execution, task
  cancellation, task steal)
* Values passed to omp_set_numtheads
* Time spent in omp single and omp master

None of this affects code compiled without stats gathering enabled,
which is the normal library build mode.

This also fixes the CMake build by linking to the standard c++ library
when building the stats library as it is a requirement.  The normal library
does not have this requirement and its link phase is left alone.

Differential Revision: http://reviews.llvm.org/D11759

llvm-svn: 244677