Sema: Fix setter visibility computation when checking key path literals.

`AbstractStorageDecl::isSettable` by itself doesn't account for access control, so we need to check `isSetterAccessibleFrom` as well. Furthermore, key paths should not get the special context-sensitive privileges direct accesses do, such as initializers being able to assign into `let` properties. Fix the type checking logic here to only create a statically writable KeyPath value when the referenced declaration has a setter visible independent of current context. Fixes SR-6667 | rdar://problem/36257266.

lib/Sema/CSSimplify.cpp

@@ -4033,7 +4033,12 @@ ConstraintSystem::simplifyKeyPathConstraint(Type keyPathTy,
         return SolutionKind::Error;
       }
       
-      if (!storage->isSettable(DC)) {
+      // See whether key paths can store to this component. (Key paths don't
+      // get any special power from being formed in certain contexts, such
+      // as the ability to assign to `let`s in initialization contexts, so
+      // we pass null for the DC to `isSettable` here.)
+      if (!storage->isSettable(nullptr)
+          || !storage->isSetterAccessibleFrom(DC)) {
         // A non-settable component makes the key path read-only, unless
         // a reference-writable component shows up later.
         capability = ReadOnly;