TableGen: Remove the cast-from-string-to-variable-reference feature

Summary:
Cast-from-string for records isn't going away, but cast-from-string for
variables is a pretty dodgy feature to have, especially when referencing
template arguments. It's doubtful that this ever worked in a reliable
way, and nobody seems to be using it, so let's get rid of it and get
some related cleanups.

Change-Id: I395ac8a43fef4cf98e611f2f552300d21e99b66a

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D44195

llvm-svn: 327844