[Concurrency] Compute the source of actor isolation in ActorIsolationRequest.

[hborla]

Every time I see a new project failure that says expression is ‘async’ but not marked with ‘await’ or other actor isolation error, and the callee is not async or explicitly annotated with a global actor attribute, I go on a hunt for where the isolation may have been inferred from. To save time searching for the source of actor isolation inference, this change records the source of actor isolation in ActorIsolationRequest.

For example:

@MainActor
protocol P {}

struct S: P {
  func f() {}
}

func call(s: S) {
  s.f()
}

The above code now produces the following diagnostics:

 error: call to main actor-isolated instance method 'f()' in a synchronous nonisolated context
 3 | 
 4 | struct S: P {
 5 |   func f() {}
   |        |- note: calls to instance method 'f()' from outside of its actor context are implicitly asynchronous
   |        `- note: main actor isolation inferred from conformance to protocol 'P'
 6 | }
 7 | 
 8 | func call(s: S) {
   |      `- note: add '@MainActor' to make global function 'call(s:)' part of global actor 'MainActor'
 9 |   s.f()
   |     `- error: call to main actor-isolated instance method 'f()' in a synchronous nonisolated context
10 | }
11 | 

The note main actor isolation inferred from conformance to protocol 'P' is new with this change.

This information is currently only used as an attached note to one error produced by the actor isolation checker, but this should be used in many more places. The source of actor isolation inference could also potentially be surfaced by SourceKit requests in the future.

Resolves: rdar://119905790

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[ktoso]

LGTM, very nice :)