DebugInfo: Remove some extra handling of abstract variables and instead rely solely on the delayed handling introduced in r210946

Now that we handle finding abstract variables at the end of the module,
remove the upfront handling and just ensure the abstract variable is
built when necessary.

In theory we could have a split implementation, where inlined variables
are immediately constructed referencing the abstract definition, and
concrete variables are delayed - but let's go with one solution for now
unless there's a reason not to.