MIR Serialization: Serialize the list of machine basic blocks with simple attributes.
This commit implements the initial serialization of machine basic blocks in a
machine function. Only the simple, scalar MBB attributes are serialized. The
reference to LLVM IR's basic block is preserved when that basic block has a name.
Reviewers: Duncan P. N. Exon Smith
Differential Revision: http://reviews.llvm.org/D10465