ASTPrinter: handle inverses in compositions
The basic inheritance clause emission in ASTPrinter operates on
InheritedEntry's, but does not canonicalize types. It's been
designed to consider an entire composition unprintable because one
member is unprintable (e.g., the protocol is not public).
This rejection is what was causing `~Copyable` in some compositions to
be missing from interface files (rdar://126090425). Fixing that is the
purpose of this patch.
What happens, then, if you mix public and nonpublic protocols in a
composition? A second facility called the InheritedProtocolCollector
later does find the public protocols, and emits extensions at the end of
the interface file to declare the additional conformances the ininitial
declaration printer missed.
We can't generally emit `~Copyable` on an extension, so the fix can't
happening there. Refactoring things so there's one source of truth about
the protocols being printed is a sizable refactoring that I will defer
for another time.
resolves rdar://126090425
