[opt-viewer] Put critical items in parallel

[opt-viewer] Put critical items in parallel

Summary:
Put opt-viewer critical items in parallel

Patch by Brian Cain!

Requires features from Python 2.7

**Performance**
Below are performance results across various configurations. These were taken on an i5-5200U (dual core + HT). They were taken with a small subset of the YAML output of building Python 3.6.0b3 with LTO+PGO. 60 YAML files.

"multiprocessing" is the current submission contents. "baseline" is as of 544f14c6b2a07a94168df31833dba9dc35fd8289 (I think this is aka r287505).

"ImportError" vs "class<...CLoader>" below are just confirming the expected configuration (with/without CLoader).

The below was measured on AMD A8-5500B (4 cores) with 224 input YAML files, showing a ~1.75x speed increase over the baseline with libYAML.  I suspect it would scale well on high-end servers.

```
**************************************** MULTIPROCESSING ****************************************
PyYAML:
        Traceback (most recent call last):
          File "<string>", line 1, in <module>
        ImportError: cannot import name CLoader
        Python 2.7.10
489.42user 5.53system 2:38.03elapsed 313%CPU (0avgtext+0avgdata 400308maxresident)k
0inputs+31392outputs (0major+473540minor)pagefaults 0swaps

PyYAML+libYAML:
        <class 'yaml.cyaml.CLoader'>
        Python 2.7.10
78.69user 5.45system 0:32.63elapsed 257%CPU (0avgtext+0avgdata 398560maxresident)k
0inputs+31392outputs (0major+542022minor)pagefaults 0swaps

PyPy/PyYAML:
        Traceback (most recent call last):
          File "<builtin>/app_main.py", line 75, in run_toplevel
          File "<builtin>/app_main.py", line 601, in run_it
          File "<string>", line 1, in <module>
        ImportError: cannot import name 'CLoader'
        Python 2.7.9 (2.6.0+dfsg-3, Jul 04 2015, 05:43:17)
        [PyPy 2.6.0 with GCC 4.9.3]
154.27user 8.12system 0:53.83elapsed 301%CPU (0avgtext+0avgdata 627960maxresident)k
808inputs+30376outputs (0major+727994minor)pagefaults 0swaps
**************************************** BASELINE        ****************************************
PyYAML:
        Traceback (most recent call last):
          File "<string>", line 1, in <module>
        ImportError: cannot import name CLoader
        Python 2.7.10
        358.08user 4.05system 6:08.37elapsed 98%CPU (0avgtext+0avgdata 315004maxresident)k
0inputs+31392outputs (0major+85252minor)pagefaults 0swaps

PyYAML+libYAML:
        <class 'yaml.cyaml.CLoader'>
        Python 2.7.10
50.32user 3.30system 0:56.59elapsed 94%CPU (0avgtext+0avgdata 307296maxresident)k
0inputs+31392outputs (0major+79335minor)pagefaults 0swaps

PyPy/PyYAML:
        Traceback (most recent call last):
          File "<builtin>/app_main.py", line 75, in run_toplevel
          File "<builtin>/app_main.py", line 601, in run_it
          File "<string>", line 1, in <module>
        ImportError: cannot import name 'CLoader'
        Python 2.7.9 (2.6.0+dfsg-3, Jul 04 2015, 05:43:17)
        [PyPy 2.6.0 with GCC 4.9.3]
72.94user 5.18system 1:23.41elapsed 93%CPU (0avgtext+0avgdata 455312maxresident)k
0inputs+30392outputs (0major+110280minor)pagefaults 0swaps

```

Reviewers: fhahn, anemet

Reviewed By: anemet

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D26967

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293261 91177308-0d34-0410-b5e6-96231b3b80d8