TableGen: Check the dynamic type of !cast<Rec>(string)

Summary:
The docs already claim that this happens, but so far it hasn't. As a
consequence, existing TableGen files get this wrong a lot, but luckily
the fixes are all reasonably straightforward.

To make this work with all the existing forms of self-references (since
the true type of a record is only built up over time), the lookup of
self-references in !cast is delayed until the final resolving step.

Change-Id: If5923a72a252ba2fbc81a889d59775df0ef31164

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D44475

llvm-svn: 327849