Add Multidef Data Structures

Add a set of data structures and members analogous to those used for
multiclass defs.  These will represent a new kind of multiclass def: a
multidef.  The idea behind the multidef is to process a list of items
and create a def record for each one inside the enclosing multiclass.
This allows the user to dynamically create a set of defs based on the
contents of a list.

llvm-svn: 141230