RFR: 8283237: CallSite should be a sealed class [v3]

[ExE Boss]

On Fri, 18 Mar 2022 22:12:10 GMT, liach <duke at openjdk.java.net> wrote:

>> Change `CallSite` to a sealed class, as `CallSite` is an abstract class which does not allow direct subclassing by users per its documentation. Since I don't have a JBS account, I posted the content for the CSR in a GitHub Gist at https://gist.github.com/150d5aa7f8b13a4deddf95969ad39d73 and wish someone can submit a CSR for me.
>
> liach has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove homemade callsite from test

The purpose of `GenManyIndyCorrectBootstrap`’s `NewInvokeSpecialCallSite` is to check that bootstrap methods work correctly with a `REF_newInvokeSpecial` method handle.

Instead, it should probably be implemented by subclassing `MutableCallSite`.

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PR: https://git.openjdk.java.net/jdk/pull/7840