Getter-only #-name accessors should throw in strict mode

[syg]

I believe the intention is to align with public properties on this. Currently,

function thisDoesntThrow() {
  class C { get foo() { return 42; } }
  let c = new C;
  c.foo = "bar";
}

function thisThrows() {
  "use strict";
  class C { get foo() { return 42; } }
  let c = new C;
  c.foo = "bar";
};

The current spec doesn't have this behavior for #foo. https://tc39.github.io/proposal-private-methods/#sec-privatefieldset returns false instead of throwing, but the caller PutValue doesn't check the return boolean.

[littledan]

Good catch, @syg. I was hoping that returning false would make the algorithm calling this throw, but that appears to not be the case. PRs welcome.