[Concurrency] Diagnose Sendable violations in captures of local functions.

[hborla]

Previously, the actor isolation checker only checked local captures of closures. This change correctly diagnoses the following code:

class NonSendable {}

func testLocalCaptures() {
  let ns = NonSendable()

  @Sendable func a2() -> NonSendable {
    return ns // error:  capture of 'ns' with non-sendable type 'NonSendable' in a `@Sendable` local function
  }
}

(Note that this PR currently contains the change for #68685)

Resolves rdar://113293502

[hborla]

@swift-ci please smoke test

[slavapestov]

Nice cleanup!