Noncopyable: fix implicit conversion error
Implicit initializers internally have "Default" parameter ownership
specifiers, which can only happen for a noncopyable type if the decl is
synthesized, since ownership is required to be specified if it were
written in the source.
There's no reason to warn for such decls, since the user can't see it
anyway. So specifically target an explicitly written "borrowing" as
being confusing, since that's the only valid case that could happen
anyway.
resolves rdar://131546153
