Add support for JSON-XPC transport for the ClangdXPC.framework

[hyp]

This patch adds support for the XPC transport layer that is used by ClangdXPC.framework.

A couple of notes:

• A lot of code between the JSON and XPC connection is the same. I think it might be possible to share it by using a common base class / protocol. @benlangmuir, WDYT?
• Tested locally, but swift-ci doesn't build the XPC framework yet, so it won't be tested in CI. I'll have to fix that too.

[benlangmuir]

Could you say a bit about the choice not to use XPC's own API for sending a message with a reply?

[hyp]

Could you say a bit about the choice not to use XPC's own API for sending a message with a reply?

It looks like send_with_reply only catches the reply if the server sends the response using the xpc_dictionary_create_reply function. Unfortunately we currently don't support that in Clangd, and we send responses back without preserving the XPC reply link. Do you think that should be fixed first?

[hyp]

Updated to use XPC data instead of string to match : https://reviews.llvm.org/D63961

[shahmishal]

The Swift project moved the default branch to main and deleted master branch, so GitHub automatically closed the PR. Please re-create the pull request with main branch.

More detail about the branch update - https://forums.swift.org/t/updating-branch-names/40412