YAML: Implement block scalar parsing.

This commit implements the parsing of YAML block scalars.
Some code existed for it before, but it couldn't parse block
scalars.

This commit adds a new yaml node type to represent the block
scalar values. 

This commit also deletes the 'spec-09-27' and 'spec-09-28' tests
as they are identical to the test file 'spec-09-26'.

This commit introduces 3 new utility functions to the YAML scanner
class: `skip_s_space`, `advanceWhile` and `consumeLineBreakIfPresent`.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D9503

llvm-svn: 237314