[Concurrency] Split up the non-Sendable diagnostics and improve wording.

[hborla]

This change splits up the Sendable diagnostics for parameters, results, and property types. The old mega-diagnostic was difficult for me to piece together based on the various diagnostic arguments. Separate diagnostics makes it a bit easier to visualize what the resulting diagnostic will say for a given set of arguments.

The wording of the error message now puts the problem -- crossing an isolation boundary -- at the center of the message instead of at the end, and attempts to clarify how the value crosses an isolation boundary. E.g. for the witness diagnostics, the value crosses an isolation boundary when calling the witness through the protocol requirement in generic code. I also eliminated some of the jargon like "implicitly asynchronous" because I don't think it's insightful for understanding this particular set of diagnostics.

For example:

class NotSendable {}

protocol P {
  func f(_: NotSendable) async
}

actor A: P {
  func f(_: NotSendable) {}
}

Currently, the compiler produces the following error:

 7 | actor A: P {
 8 |   func f(_: NotSendable) {}
   |        `- error: non-sendable type 'NotSendable' in parameter of the protocol requirement satisfied by actor-isolated instance method 'f' cannot cross actor boundary
 9 | }
10 | 

The above error is not ideal for 2 reasons:

1. The problem -- the fact that the value crosses an isolation boundary -- is at the very end of the message
2. It's not clear how an isolation boundary is crossed

With this change, the following error is produced:

 7 | actor A: P {
 8 |   func f(_: NotSendable) {}
   |        `- error: non-sendable parameter type 'NotSendable' cannot be sent from caller of protocol requirement 'f' into actor-isolated implementation
 9 | }
10 | 

This change (hopefully) clarifies that the problem occurs when you call the method through the nonisolated protocol requirement, because an isolation boundary is crossed when calling through to the underlying actor-isolated implementation.

This change does not add any additional information to the diagnostics, but it'd be valuable to show both the source and destination isolation.

Resolves: rdar://133215909

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[ktoso]

Looks like a good improvement, the case remains pretty confusing but this is better than before :)

[hborla]

the case remains pretty confusing but this is better than before :)

Yeah agreed, I find the protocol witness and overriding cases particularly hard to explain in a one-line diagnostic. I suppose I should write educational notes for all of these!