TableGen: Generalize record types to fix typeIsConvertibleTo et al.

TableGen: Generalize record types to fix typeIsConvertibleTo et al.

Summary:
Allow RecordRecTy to represent the type "subclass of N superclasses",
where N may be zero. Furthermore, generate RecordRecTy instances only
with actual classes in the list.

Keeping track of multiple superclasses is required to resolve the type
of a list correctly in some cases. The old code relied on the incorrect
behavior of typeIsConvertibleTo, and an earlier version of this change
relied on a modified ordering of superclasses (it was committed in
r325884 and then reverted because unfortunately some of clang-tblgen's
backends depend on the ordering).

Previously, the DefInit for each Record would have a RecordRecTy of
that Record as its type. Now, all defs with the same superclasses will
share the same type.

This allows us to be more consistent about type checks involving records:

- typeIsConvertibleTo actually requires the LHS to be a subtype of the
  RHS

- resolveTypes will return the least supertype of given record types in
  all cases

- different record types in the two branches of an !if are handled
  correctly

Add a test that used to be accepted without flagging the obvious type
error.

Change-Id: Ib366db1a4e6a079f1a0851e469b402cddae76714

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43680

llvm-svn: 326783