When linking `save` assemblis with `-b true` (linking debug info), we preserve
all debug info (and don't sweep the debug info like we do for `link` assemblies).

With this change, we now take into account type references that are only referenced
by the debug info, preserving any assembly refs needed by them.

This adds test infra to walk the type references in the output assembly, and
check that they reference assembly refs that also exist in the output.

Includes some test infra changes that were ultimately not necessary for the repro,
but were useful, to allow passing multiple additional compiler args when compiling
testcase dependencies.

This should be adequately covered by NoLinkedOutputAttribute