Error structs synthesized by ClangImporter can be renamed using SWIFT_NAME() to syntactically appear anywhere in the type hierarchy with any name, but they should always be mangled as `__C_Synthesized.related decl ‘e’ of <Objective-C enum name>`. Unforunately, when SWIFT_NAME() was used to nest the error struct inside another type, an ASTMangler bug would cause it to be mangled as `<parent type>.related decl ‘e’ of <Objective-C enum name>`, and an ASTDemangler bug would also require a valid parent type. This created a mismatch between the compiler’s and runtime’s manglings which caused crashes when you tried to match the imported error struct in a `catch`.
This PR corrects the compiler bugs so that it generates the mangling the runtime expects. This is theoretically ABI-breaking, but as far as I can determine nobody has shipped the incorrectly mangled names, presumably because they crash when you try to use them.
Fixes <rdar://problem/48040880>.
When an enum is imported as an error, the imported type itself becomes
a nested type 'Code' of its wrapper type, so you get a type like
'MyError.Code'. However in the mangling grammar the type is a
child of a module and not another type.
This was tripping up TypeDecoder, which would check parent types for
validity and throw out the demangling since it looked invalid.
However since this case really is valid, just skip the whole parent
type mess when the type is imported and non-generic.
