[region-isolation] Change the main race error to be "transferring {could,may} cause a race"

As I was updating tests in the previous commit, I noticed that this would read better to me.

Author: gottesmm

include/swift/AST/DiagnosticsSIL.def

@@ -971,7 +971,7 @@ NOTE(regionbasedisolation_isolated_since_in_same_region_basename, none,
 //
 
 ERROR(regionbasedisolation_named_transfer_yields_race, none,
-      "transferring %0 could cause a race",
+      "transferring %0 may cause a race",
       (Identifier))
 NOTE(regionbasedisolation_named_info_transfer_yields_race, none,
      "%0 is transferred from %1 caller to %2 callee. Later uses in caller could race with potential uses in callee",

test/Concurrency/experimental_feature_strictconcurrency.swift

@@ -31,7 +31,7 @@ func iterate(stream: AsyncStream<Int>) async {
   // FIXME: Region isolation should consider a value from a 'nonisolated(unsafe)'
   // declaration to be in a disconnected region
 
-  // expected-region-isolation-warning @+3 {{transferring 'it' could cause a race}}
+  // expected-region-isolation-warning @+3 {{transferring 'it' may cause a race}}
   // expected-region-isolation-note @+2 {{'it' is transferred from main actor-isolated caller to nonisolated callee. Later uses in caller could race with potential uses in callee}}
   // expected-region-isolation-note @+1 {{access here could race}}
   while let element = await it.next() {

test/Concurrency/isolated_captures.swift

@@ -39,7 +39,7 @@ class NotSendable {
   let ns = NotSendable(x: 0)
   MyActor.ns = ns
 
-  // expected-region-isolation-warning @+2 {{transferring 'ns' could cause a race}}
+  // expected-region-isolation-warning @+2 {{transferring 'ns' may cause a race}}
   // expected-region-isolation-note @+1 {{transferring global actor 'MyActor'-isolated 'ns' to global actor 'YourActor'-isolated callee could cause races between global actor 'YourActor'-isolated and global actor 'MyActor'-isolated uses}}
   await { @YourActor in
     // expected-complete-warning@+1 {{capture of 'ns' with non-sendable type 'NotSendable' in an isolated closure; this is an error in Swift 6}}

test/Concurrency/sendable_checking.swift

@@ -251,12 +251,12 @@ final class NonSendable {
   func call() async {
     await update()
     // expected-targeted-and-complete-warning @-1 {{passing argument of non-sendable type 'NonSendable' into main actor-isolated context may introduce data races}}
-    // expected-tns-warning @-2 {{transferring 'self' could cause a race}}
+    // expected-tns-warning @-2 {{transferring 'self' may cause a race}}
     // expected-tns-note @-3 {{transferring nonisolated 'self' to main actor-isolated callee could cause races between main actor-isolated and nonisolated uses}}
 
     await self.update()
     // expected-targeted-and-complete-warning @-1 {{passing argument of non-sendable type 'NonSendable' into main actor-isolated context may introduce data races}}
-    // expected-tns-warning @-2 {{transferring 'self' could cause a race}}
+    // expected-tns-warning @-2 {{transferring 'self' may cause a race}}
     // expected-tns-note @-3 {{transferring nonisolated 'self' to main actor-isolated callee could cause races between main actor-isolated and nonisolated uses}}
 
     _ = await x
@@ -275,7 +275,7 @@ func testNonSendableBaseArg() async {
   let t = NonSendable() // expected-tns-note {{variable defined here}}
   await t.update()
   // expected-targeted-and-complete-warning @-1 {{passing argument of non-sendable type 'NonSendable' into main actor-isolated context may introduce data races}}
-  // expected-tns-warning @-2 {{transferring 't' could cause a race}}
+  // expected-tns-warning @-2 {{transferring 't' may cause a race}}
   // expected-tns-note @-3 {{'t' is transferred from nonisolated caller to main actor-isolated callee. Later uses in caller could race with potential uses in callee}}
 
   _ = await t.x
@@ -295,13 +295,13 @@ func callNonisolatedAsyncClosure(
 ) async {
   await g(ns)
   // expected-targeted-and-complete-warning @-1 {{passing argument of non-sendable type 'NonSendable' outside of main actor-isolated context may introduce data races}}
-  // expected-tns-warning @-2 {{transferring 'ns' could cause a race}}
+  // expected-tns-warning @-2 {{transferring 'ns' may cause a race}}
   // expected-tns-note @-3 {{transferring main actor-isolated 'ns' to nonisolated callee could cause races between nonisolated and main actor-isolated uses}}
 
   let f: (NonSendable) async -> () = globalSendable // okay
   await f(ns)
   // expected-targeted-and-complete-warning@-1 {{passing argument of non-sendable type 'NonSendable' outside of main actor-isolated context may introduce data races}}
-  // expected-tns-warning @-2 {{transferring 'ns' could cause a race}}
+  // expected-tns-warning @-2 {{transferring 'ns' may cause a race}}
   // expected-tns-note @-3 {{transferring main actor-isolated 'ns' to nonisolated callee could cause races between nonisolated and main actor-isolated uses}}
 }