[region-isolation] Convert more diagnostics from type isolation form to named variable form

[gottesmm]

Just chopping off parts of a larger PR that can exist independently.

In this PR, I eliminate a ton more of the cases where we failed to identify a value name resulting in us emitting a type isolation error. Example:

// After
func letSendableTrivialClassFieldTest() async {
  let test = ClassFieldTests()
  await transferToMain(test) // expected-tns-warning {{transferring 'test' may cause a race}}
  // expected-tns-note @-1 {{'test' is transferred from nonisolated caller to main actor-isolated callee. Later uses in caller could race with potential uses in callee}}
  // expected-complete-warning @-2 {{passing argument of non-sendable type 'ClassFieldTests' into main actor-isolated context may introduce data races}}
  _ = test.letSendableTrivial
  useValue(test) // expected-tns-note {{access here could race}}
}

from,

// Before
func letSendableTrivialClassFieldTest() async {
  let test = ClassFieldTests()
  await transferToMain(test) // expected-tns-warning {{transferring value of non-Sendable type 'ClassFieldTests' from nonisolated context to main actor-isolated context; later accesses could race}}
  // expected-complete-warning @-1 {{passing argument of non-sendable type 'ClassFieldTests' into main actor-isolated context may introduce data races}}
  _ = test.letSendableTrivial
  useValue(test) // expected-tns-note {{access here could race}}
}

I still need to change that specific diagnostic to use IsolationRegionInformation rather than pure callee information, but this an improvement (and more importantly just what I needed to add to do some other work). Once I am able to loop back around that would cause the note diagnostic to be transformed as follows:

  // expected-tns-note @-1 {{'test' is transferred from nonisolated caller to main actor-isolated callee. Later uses in caller could race with potential uses in callee}}

->

  // expected-tns-note @-1 {{disconnected 'test' is transferred to main actor-isolated callee. Later uses in caller could race with actor-isolated uses in callee}}

[gottesmm]

@swift-ci smoke test