[TableGen] Diagnose undefined fields when generating searchable tables

Summary:
Previously TableGen would crash trying to print the undefined value as
an integer.

Change-Id: I3900071ceaa07c26acafb33bc49966d7d7a02828

Reviewers: nhaehnle

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74210