swift234

Improve diagnostic text for @available(renamed:).

We don't want to show the funny "getter:Foo.bar(self:)" syntax to
developers, so whenever possible be a little more explicit.

    'foo' has been replaced by 'X.newFoo'
    'bar(x:)' has been replaced by property 'X.bar'
    'baz(x:y:)' has been replaced by instance method 'X.baz(y:)'

(We do run up against the limitation of a string -- this diagnostic
does not do any lookup to find out if the resulting decl actually exists.)