YAML: Add support for literal block scalar I/O.

This commit gives the users of the YAML Traits I/O library 
the ability to serialize scalars using the YAML literal block 
scalar notation by allowing them to implement a specialization 
of the `BlockScalarTraits` struct for their custom types.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D9613

llvm-svn: 237404