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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237404 91177308-0d34-0410-b5e6-96231b3b80d8