Allow duplicate __proto__ keys in patterns, simple case (#6705)

[alejo90]

Q	A
Fixed Issues?	Fixes #6705
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?	No
License	MIT

This is a fix for an error being thrown when there is a duplicate __proto__ in a destructuring pattern. It only fixes the simple case when the pattern is immediately followed by a =. I renamed the checkPropClash function to checkDuplicatedProto, as advised in the issue. Instead of relying exclusively on a boolean flag to determine whether __proto__ had appeared before or not, I store the amount of times it has appeared and the position of the first duplicated occurrence. In parseObj, I check for the existence of the first duplicated occurrence property I stored before and raise an error if it's there and the next token is not a =.
Please let me know if there are any concerns or improvements I can make. I'm thinking about the more complicated case as well but I'll need a little more time with the code before I can tackle it.
Thanks @nicolo-ribaudo for the hints on how to get started.

[nicolo-ribaudo]

We also need to update the test262 whitelist. Could you run make bootstrap-test262 and make test-test262-update-whitelist?

[nicolo-ribaudo]

Thanks!

[JLHwung]

In this PR we have fixed the simple case of #6705, I suggest we submit a new issue to cover the following cases:

// argument destructuring
({ __proto__: x, __proto__: y }) => {}
// forX destructuring
for ({ __proto__: x, __proto__: y } of []);
// nested destructuring
({ a: { __proto__: x, __proto__: y } } = { a: {} }) 

[nicolo-ribaudo]

@JLHwung I have repoened the issue