swift234

[QoI] Improve diagnostics when putting a class bound in a generic signature

When declaring a nominal type:

struct Weak<T> {
  weak var value: T
}

The diagnostic might mislead the developer to adding ': class' literally
to the 'T' in the generic parameters for `Weak`:

"'weak' may not be applied to non-class-bound protocol 'T'; consider
adding a class bound"

This is misleading in two ways: 1, 'T' isn't necessarily a protocol
(this patch generalizes that part of the message) and 2, you can't put
`: class` in the generic parameter list for `Weak`. In addition, the
stray class constraint causes diagnostic spew that also hides the issue.

Also provide a fix-it to constrain with 'AnyObject', which is probably
what the devloper means in this case.

rdar://problem/25481209