Recover gracefully when deserializing invalid YAML input.
Fixes http://llvm.org/PR16221, http://llvm.org/PR15927
Phabricator: http://llvm-reviews.chandlerc.com/D1236

Patch by Andrew Tulloch!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195016 91177308-0d34-0410-b5e6-96231b3b80d8