If we only emit an opaque reflection record for a struct or class, then we can't reflect its fields. We failed both to clear the "is reflectable" bit in the context descriptor for non-reflectable structs, and to check for the bit before trying to present a struct's fields as children in the runtime. rdar://problem/41274260
The old-style mirrors were the basis for non-custom, reflection based Mirrors. As part of removing old-style mirrors, reflection based Mirrors are reimplemented without them. Reflection.mm is deleted and replaced with ReflectionMirror.mm, which borrows a chunk of the old code but presents a simpler API to the Swift side. ReflectionMirror.swift then uses that API to implement a reflection-based Mirror init.
rdar://problem/20356017
This makes them consistent no matter what shenanigans are pulled by
the importer, particularly NS_ENUM vs. NS_OPTIONS and NS_SWIFT_NAME.
The 'NSErrorDomain' API note /nearly/ works with this, but the
synthesized error struct is still mangled as a Swift declaration,
which means it's not rename-stable. See follow-up commits.
The main place where this still falls down is NS_STRING_ENUM: when
this is applied, a typedef is imported as a unique struct, but without
it it's just a typealias for the underlying type. There's also still a
problem with synthesized conformances, which have a module mangled
into the witness table symbol even though that symbol is linkonce_odr.
rdar://problem/31616162
