MIR Serialization: Serialize MBB successors.

This commit implements serialization of the machine basic block successors. It
uses a YAML flow sequence that contains strings that have the MBB references.
The MBB references in those strings use the same syntax as the MBB machine
operands in the machine instruction strings.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10699

llvm-svn: 241093