Permit Symbol as WeakMap key and WeakSet entry. Closes gh-2850

[rwaldron]

For changes that are explicitly relevant to "Permit Symbol as WeakMap key and WeakSet entry", see commit 1.

[ExE-Boss]

This needs to be updated so that it also checks that symbols are valid WeakRef targets.

[rwaldron]

@leobalter does your change in https://github.com/tc39/ecma262/compare/main...leobalter:symbol-as-weakmap-key cover @ExE-Boss's comment? I was expecting to see something like:

- 2. If Type(target) is not Object, throw a TypeError exception.
+ 2. If HasIdentity(target) is false, throw a TypeError exception.

Am I misunderstood?