This opens the door to introducing a YAML outputter which can be
used for machine consumption. Currently the yaml output style
is unimplemented and returns an error if you try to use it.
Reviewed By: rnk, ruiu
Differential Revision: http://reviews.llvm.org/D20967
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271712 91177308-0d34-0410-b5e6-96231b3b80d8
