TableGen: Explicitly forbid some nestings of class, multiclass, and foreach

These previously all failed one way or another, but now we produce a more
helpful error message.

Change-Id: I8ffd2e87c8e35a5134c3be289e0a1fecaa2bb8ca

Differential revision: https://reviews.llvm.org/D44115

llvm-svn: 327497