[5.9][Macros] Unify PluginMessages.swift

[rintaro]

cherry-pick #67025 into release/5.9

swift-syntax change swiftlang/swift-syntax#1866

• Explanation: ASTGen in swift repo and SwiftCompilerPluginMessageHandling in swift-syntax used to have separate PluginMessages.swift. When we change the executable plugin IPC message format, we needed to change both files exactly the same way. That was confusing and there was no benefit of having separate files in 2 repositories. Instead, expose that message types from swift-syntax and use it from swift repo. So that we have the single source of truth in swift-syntax
• Scope: Executable plugin IPC messaging.
• Risk: Low. ASTGen now import the module in swift-syntax repo which is already built/installed in the toolchain
• Testing: Passes current test suites
• Issue: rdar://111538595
• Reviewer: Doug Gregor (@DougGregor)

[rintaro]

swiftlang/swift-syntax#1866
@swift-ci Please test