[opt-viewer] Add support for libYAML for faster parsing

This results in a speed-up of over 6x on sqlite3.

Before:

$ time -p /org/llvm/utils/opt-viewer/opt-viewer.py ./MultiSource/Applications/sqlite3/CMakeFiles/sqlite3.dir/sqlite3.c.opt.yaml html
  real 415.07
  user 410.00
  sys 4.66

After with libYAML:

$ time -p /org/llvm/utils/opt-viewer/opt-viewer.py ./MultiSource/Applications/sqlite3/CMakeFiles/sqlite3.dir/sqlite3.c.opt.yaml html
  real 63.96
  user 60.03
  sys 3.67

I followed these steps to get libYAML working with PyYAML: http://rmcgibbo.github.io/blog/2013/05/23/faster-yaml-parsing-with-libyaml/

llvm-svn: 286942