YAML: Assign a value returned by the default constructor to the value in an optional mapping.
This commit ensures that a value that's passed into YAML's IO mapOptional method
is going to be assigned a value returned by the default constructor for that
value's type when the appropriate key is not present in the YAML mapping.
Reviewers: Duncan P. N. Exon Smith
Differential Revision: http://reviews.llvm.org/D10492