[5.1] [CSDiagnostics] Improving the fix-it for defining computed properties

[theblixguy]

Cherry pick of #23500.

This PR offers an improvement when the user is trying to define a computed property that is initialised with a closure. For example:

class Foo {
  var something: Int = { return 0 }
}

We currently emit an error diagnostic for the above, with a fix-it to add ():

Function produces expected type 'Int'; did you mean to call it with '()'?

In case of extensions, we emit:

Extensions must not contain stored properties

With this change, we will offer another fix-it to convert it into a computed property, by removing the = and if the property is immutable, then also changing the let to a var.

Resolves SR-9267.

[theblixguy]

cc @xedin

[xedin]

@swift-ci please test