Sema: Fix UB in NonisolatedNonsendingByDefault migration diagnosis

The diagnostic can outlive the locally constructed attribute, which was
passed by pointer, if there is an active `DiagnosticTransaction`.

(cherry picked from commit f4e49d5)

Author: AnthonyLatsis

include/swift/AST/DiagnosticsSema.def

@@ -8634,23 +8634,23 @@ GROUPED_WARNING(
     attr_execution_nonisolated_behavior_will_change_decl, NonisolatedNonsendingByDefault,
     none,
     "feature '%0' will cause nonisolated async %kindbase1 to run on the "
-    "caller's actor; use %2 to preserve behavior",
-    (StringRef, const AbstractFunctionDecl *, DeclAttribute))
+    "caller's actor; use '@concurrent' to preserve behavior",
+    (StringRef, const AbstractFunctionDecl *))
 
 GROUPED_WARNING(
     attr_execution_nonisolated_behavior_will_change_closure,
     NonisolatedNonsendingByDefault, none,
     "feature '%0' will cause nonisolated async closure to run on the caller's "
-    "actor; use %1 to preserve behavior",
-    (StringRef, DeclAttribute))
+    "actor; use '@concurrent' to preserve behavior",
+    (StringRef))
 
 GROUPED_WARNING(
     attr_execution_nonisolated_behavior_will_change_typerepr,
     NonisolatedNonsendingByDefault, none,
     "feature '%0' will cause nonisolated async function type to be treated as "
-    "specified to run on the caller's actor; use %1 to preserve "
+    "specified to run on the caller's actor; use '@concurrent' to preserve "
     "behavior",
-    (StringRef, DeclAttribute))
+    (StringRef))
 
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/Sema/NonisolatedNonsendingByDefaultMigration.cpp

@@ -168,21 +168,21 @@ void NonisolatedNonsendingByDefaultMigrationTarget::diagnose() const {
     ctx.Diags
         .diagnose(functionDecl->getLoc(),
                   diag::attr_execution_nonisolated_behavior_will_change_decl,
-                  featureName, functionDecl, &attr)
+                  featureName, functionDecl)
         .fixItInsertAttribute(
             decl->getAttributeInsertionLoc(/*forModifier=*/false), &attr);
   } else if (closure) {
     ctx.Diags
         .diagnose(closure->getLoc(),
                   diag::attr_execution_nonisolated_behavior_will_change_closure,
-                  featureName, &attr)
+                  featureName)
         .fixItAddAttribute(&attr, closure);
   } else {
     ctx.Diags
         .diagnose(
             functionRepr->getStartLoc(),
             diag::attr_execution_nonisolated_behavior_will_change_typerepr,
-            featureName, &attr)
+            featureName)
         .fixItInsertAttribute(functionRepr->getStartLoc(), &attr);
   }
 }