[TableGen] FieldInit: improve assertion message

Summary:
When fiddling with sched profiles, especially creating new ones, it's amazingly easy
to end up with malformed .td that crashes tablegen, without explanation of the bug.
This changes the most common assertion i have encountered to dump enough information
to be able to fix the .td

Split of from D63628

Reviewers: RKSimon, craig.topper, nhaehnle

Reviewed By: craig.topper

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65790

llvm-svn: 368060