[Verifier] Stop accepting broken DIGlobalVariable(s).

The code wasn't yelling at the user when there's a reference
from a DIGlobalVariableExpression. Thanks to Adrian for the
reduced testcase. Fixes PR34672.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314069 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed