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

llvm-svn: 239972