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

llvm-svn: 240145