[rbi] Convert mutable box sending closure diagnostic to use modifiable instead of accessible in error message.

This emphasizes that the problem is that the mutable value could be modified in
multiple isolation domains which is clearer to the user.

I came up with this diagnostic in the previous commit for isolated partial
applies, so it makes sense to also be consistent with sending closures.

Author: gottesmm

include/swift/AST/DiagnosticsSIL.def

@@ -1098,7 +1098,7 @@ NOTE(regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_value_
      "closure captures %0 which is accessible to code in the current task",
      (DeclName))
 NOTE(regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_boxed_value_task_isolated, none,
-     "closure captures reference to mutable %kind0 which is accessible to code in the current task",
+     "closure captures reference to mutable %kind0 which remains modifiable by code in the current task",
      (const ValueDecl *))
 NOTE(regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_value_region, none,
      "closure captures %1 which is accessible to %0 code",

test/Concurrency/transfernonsendable.swift

@@ -1794,7 +1794,7 @@ extension MyActor {
 func nonSendableAllocBoxConsumingParameter(x: consuming SendableKlass) async throws {
   try await withThrowingTaskGroup(of: Void.self) { group in
     group.addTask { // expected-warning {{passing closure as a 'sending' parameter risks causing data races between code in the current task and concurrent execution of the closure}}
-      useValue(x) // expected-note {{closure captures reference to mutable parameter 'x' which is accessible to code in the current task}}
+      useValue(x) // expected-note {{closure captures reference to mutable parameter 'x' which remains modifiable by code in the current task}}
     }
 
     try await group.waitForAll()
@@ -1807,7 +1807,7 @@ func nonSendableAllocBoxConsumingVar() async throws {
 
   try await withThrowingTaskGroup(of: Void.self) { group in
     group.addTask { // expected-warning {{passing closure as a 'sending' parameter risks causing data races between code in the current task and concurrent execution of the closure}}
-      useValue(x) // expected-note {{closure captures reference to mutable var 'x' which is accessible to code in the current task}}
+      useValue(x) // expected-note {{closure captures reference to mutable var 'x' which remains modifiable by code in the current task}}
     }
 
     try await group.waitForAll()
@@ -1865,7 +1865,7 @@ func testFunctionIsNotEmpty(input: SendableKlass) async throws {
   var result: [SendableKlass] = []
   try await withThrowingTaskGroup(of: Void.self) { taskGroup in // expected-warning {{no calls to throwing functions occur within 'try' expression}}
     taskGroup.addTask { // expected-warning {{passing closure as a 'sending' parameter risks causing data races between code in the current task and concurrent execution of the closure}}
-      result.append(input) // expected-note {{closure captures reference to mutable var 'result' which is accessible to code in the current task}}
+      result.append(input) // expected-note {{closure captures reference to mutable var 'result' which remains modifiable by code in the current task}}
     }
   }
 }