DebugInfo: Reference abstract definitions from variables in concrete definitions that preceed their first inline definition.

Rather than relying on abstract variables looked up at the time the
concrete variable is created, look them up at the end of the module to
ensure they're referenced even if they're created after the concrete
definition. This completes/matches the work done in r209677 to handle
this for the subprograms themselves.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210946 91177308-0d34-0410-b5e6-96231b3b80d8