TableGen: Get rid of Init::getFieldInit

Summary:
FieldInit will just rely on the standardized resolving mechanism to give
us DefInits for folding, thus simplifying the code.

Unlike the removal of resolveListElementReference, this shouldn't have
performance implications, because DefInits do not recurse inside their
record.

Change-Id: Id4544c774c9d9ee92f293615af6ecff706453f21

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43563

llvm-svn: 326060