Make a single 'PluginRegistry' and share it between SwiftASTManager,
IDEInspectionInstance, and CompileInstance. And inject the plugin
registry to ASTContext right after 'CompilerInstance.setup()'
That way, all sema-capable ASTContext in SourceKit share a single
PluginRegistry.
These libraries formed a strongly connected component in the CMake build graph. The weakest link I could find was from IDE to FrontendTool and Frontend, which was necessitated by the `CompileInstance` class (https://github.com/apple/swift/pull/40645). I moved a few files out of IDE into a new IDETools library to break the cycle.
