[TableGen] Emit more helpful error messages on empty type set

I have seen this error quite frequently in our out-of-tree CHERI backends
and the lack of location information sometimes makes it quite difficult
to track down the actual source of the error.
This patch changes the llvm_unreachable() to a PrintFatalError() so that
tablegen prints a stack of source locations.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D99468