[Sema]Set IsSending flag to subscript.getter if needed

Author: stzn

lib/AST/Decl.cpp

@@ -10607,6 +10607,9 @@ AccessorDecl *AccessorDecl::createParsed(
       // The cloned parameter is implicit.
       param->setImplicit();
 
+      if (subscriptParam->isSending())
+        param->setSending();
+
       newParams.push_back(param);
     }
   }

test/Concurrency/sending_witness_subscript.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend -emit-sil -swift-version 6 %s -o /dev/null -verify
+
+class NonSendableKlass {}
+
+protocol P {
+  subscript(_: sending NonSendableKlass) -> sending NonSendableKlass { get }
+}
+
+struct S: P {
+  subscript(_: sending NonSendableKlass) -> sending NonSendableKlass { NonSendableKlass() }
+}