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.
Some of the patterns in the new test header are special-cased by
ClangImporter (for example, the names of typedefs become names of
imported structs).
Nested structs have been historically imported in Swift according to C
rules, that is, structs lexically declared nested in other structs are
imported into the global namespace anyway. For the same reason, C
structs have been mangled in Swift names as if they are declared in the
global namespace. These rules are problematic when importing C++, see
https://bugs.swift.org/browse/SR-12803.
