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.
This imports const members of C++ structs/classes stored properties with
an inaccessible setter.
Note that in C++ there are ways to change the values of const members,
so we don't use `WriteImplKind::Immutable` storage.
Resolves: [SR-12463](https://bugs.swift.org/browse/SR-12463)
