If possible, add imported members to the StructDecl's LookupTable rather than adding them directly as members. This will fix the issues with ordering that #39436 poorly attempted to solve during IRGen.
This also allows us to break out most of the test changes from #39436.
The pattern:
struct X { friend struct Y; }; struct Y {};
is fairly common. But before this commit we would crash while attempting
to add "Y" as a child of "X". This commit simply checks if the child
record is a declaration or definition. If the former, it bails (and the
"child" record will be imported where it's defined).
Rather than skipping non-definitions, we should just check whether we've
already seen this decl. This not only fixes the specific problem with
class templates but also is a more general fix for other sub decls.
