TableGen: Allow arbitrary list values as ranges of foreach

The changes to FieldInit are required to make field references (Def.field)
work inside a ForeachDeclaration: previously, Def.field wasn't resolved
immediately when Def was already a fully resolved DefInit.

Change-Id: I9875baec2fc5aac8c2b249e45b9cf18c65ae699b
llvm-svn: 327120