Previously V8 was reusing the error fur duplicate declarations, using
the private name for class fields or the class name for class methods
as the redeclared identifier.

    class A { constructor(o) { return o } }
    class B extends A { #x }
    class C extends A { #x() {} }
    let D = (0, class extends A { #x() {} });

    new B(new B({})) // Identifier '#x' has already been declared
    new C(new C({})) // Identifier 'C' has already been declared
    new D(new D({})) // Identifier '' has already been declared

This patch changes it to use error messages that better explain what's
happening:

    new B(new B({})) // Cannot initialize #x twice on the same object
    new C(new C({})) // Cannot initialize private methods of
                     // class C twice on the same object
    new D(new D({})) // Cannot initialize private methods of
                     // class anonymous twice on the same object

I initially tried to use the same message for both fields and methods,
but the problem with that is that when initializing fields we only
have access to the field name, while when initializing methods we only
have access to the class name (using the "private brand" symbol).
However, almost all the error messages are different for private fields
and for methods so this shouldn't be a problem.

Bug: v8:12042
Change-Id: Iaa50c16e4fa5c0646ad9ef2aa7e65bb649b3fce2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3078362Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Joyee Cheung <joyee@igalia.com>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76279}