[PBQP] Add support for graph-level metadata to the PBQP graph. This will be used

in the future to attach useful information about the PBQP graph (e.g. the
associated MachineFunction, pointers to regalloc passes) to the graph itself,
making that information accessible to the solver. This should also allow the
PBQPBuilder interface to be simplified.

llvm-svn: 218848