Fix bitcode upgrade for DIGlobalVariables with a var: field.

This is a follow-up to https://reviews.llvm.org/D29349.  It turns out
that NeedUpgradeToDIGlobalVariableExpression is always necessary when
we encountered a version==0 record because it may always be referenced
via a list of globals in a DICompileUnit. My tests weren't good enough
to catch this though. To trigger this case, we need much older bitcode
produced by LLVM around version 3.7.

<rdar://problem/30404262>

Differential Revision: https://reviews.llvm.org/D29693

llvm-svn: 294488