[opt-viewer] For single-process, fall back on map instead of Pool.map

This allows for nicer backtrace and debugging when -j1 is passed:

  $ opt-viewer.py CMakeFiles/LLVMScalarOpts.dir/LoopVersioningLICM.cpp.opt.yaml html
  Traceback (most recent call last):
    File "/org/llvm/utils/opt-viewer/opt-viewer.py", line 405, in <module>
      generate_report(pmap, all_remarks, file_remarks, args.source_dir, args.output_dir)
    File "/org/llvm/utils/opt-viewer/opt-viewer.py", line 362, in generate_report
      pmap(_render_file_bound, file_remarks.items())
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 251, in map
      return self.map_async(func, iterable, chunksize).get()
    File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 567, in get
      raise self._value
  Exception: blah

  $ opt-viewer.py -j 1 CMakeFiles/LLVMScalarOpts.dir/LoopVersioningLICM.cpp.opt.yaml html
  Traceback (most recent call last):
    File "/org/llvm/utils/opt-viewer/opt-viewer.py", line 405, in <module>
      generate_report(pmap, all_remarks, file_remarks, args.source_dir, args.output_dir)
    File "/org/llvm/utils/opt-viewer/opt-viewer.py", line 362, in generate_report
      pmap(_render_file_bound, file_remarks.items())
    File "/org/llvm/utils/opt-viewer/opt-viewer.py", line 317, in _render_file
      SourceFileRenderer(source_dir, output_dir, filename).render(remarks)
    File "/org/llvm/utils/opt-viewer/opt-viewer.py", line 168, in __init__
      raise Exception("blah")
  Exception: blah

llvm-svn: 295080