[CoroutineAccessors] Don't synthesize overrides.

Such storage decls don't actually have entries in the wtable.

Author: nate-chandler

lib/Sema/TypeCheckStorage.cpp

@@ -950,6 +950,9 @@ bool AccessorDecl::doesAccessorHaveBody() const {
     if (!requiresFeatureCoroutineAccessors(accessor->getAccessorKind())) {
       return false;
     }
+    if (storage->getOverrideLoc()) {
+      return false;
+    }
     return accessor->getStorage()
         ->requiresCorrespondingUnderscoredCoroutineAccessor(
             accessor->getAccessorKind(), accessor);

test/TBD/coroutine_accessors.swift

@@ -17,3 +17,7 @@ public protocol P {
 
   var s: S { read set }
 }
+
+public protocol Q : P {
+  override var s: S { read set }
+}