The diagnostics engine has some code to pretty-print a declaration when
there is no source location for that declaration. The declaration is
pretty-printed into a source buffer, and a source location into that
buffer is synthesizes. This applies to synthesized declarations as well
as those imported from Swift modules (without source code) or from Clang.
Reimplement this pretty-printing for declarations as a request. In
doing so, change the manner in which we do the printing: the
diagnostics engine printed the entire enclosing type into a buffer
whose name was the module + that type. This meant that the buffer was
shared by every member of that type, but also meant that we would end
up deserializing a lot of declarations just for printing and
potentially doing a lot more work for these diagnostics.
Overlays are specifically allowed to do magic things that regular Swift
modules are not. In this case, the UIImage and NSImage overlays have
extensions that allow them to conform to `_ExpressibleByImageLiteral`.
This, unfortunately, means that subclassing these classes with the
overlays present is impossible as one must override the required
initializers, but one cannot override these initializers since they are
declared in an extension.
While we cannot correct the overlays without breaking ABI, we can at
least make sure we don't attempt to stick @objc into them when users
have a go at subclassing these classes.
Resolves rdar://59610201
