Revert "DebugInfo: Don't put types in type units if they reference internal linkage types"

This reverts commit ab475633.

Breaks some cases, including this:

namespace {
template <typename> struct a {};
} // namespace
class c {
  c();
};
class b {
  b();
  a<c> ax;
};
b::b() {}
c::c() {}

By producing a reference to a type unit for "c" but not producing the type unit.