YAML: Fix the output of sequences that contain flow sequences.

This patch fixes a bug where the YAML Output class emitted
a sequence of flow sequences without the '-' characters.
Before:
  
  seq:
    [ a, b ]
    [ c, d ]

After:

  seq:
    - [ a, b ]
    - [ c, d ]


Reviewers: Justin Bogner

Differential Revision: http://reviews.llvm.org/D9206

llvm-svn: 236329