[TableGen] Improve error reporting

When searching for a resource unit, use the reference location instead of
the definition location in case of an error.

Differential revision: https://reviews.llvm.org/D40263

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318803 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed