To support nested structs, we emit type aliases in the outer class.
Unfortunately, we emitted these type aliases unconditionally, even if
the actualy nested struct was not emitted to the reverse interop header
(due to visibility or the construct being unsupported). This PR fixed
this issue by checking first if the nested entity should be included in
the reverse interop header.
rdar://141688074
It is really involved to change how methods and classes are emitted into
the header so this patch introduces the impression of nested structs
through using statements and still emits the structs themselves as top
level structs. It emits them in their own namespace to avoid name
collisions. This patch also had to change some names to be fully
qualified to avoid some name lookup errors in case of nested structs.
Moreover, nesting level of 3 and above requires C++17 because it relies
on nested namespaces. Only nested structs are supported, not nested
classes.
Since this patch is already started to grow quite big, I decided to put
it out for reviews and plan to address some of the shortcomings in a
follow-up PR.
rdar://118793469
