[DebugInfo] Deduplicate debug info limiting logic

We should be doing the same checks when a type is completed as we do
when a complete type is used during emission. Previously, we duplicated
the logic, and it got out of sync. This could be observed with
dllimported classes.

Also reduce a test case for this slightly.

Implementing review feedback from David Blaikie on r281057.

llvm-svn: 281278