[Macros] Add 'LibraryPluginProvider'

[rintaro]

LibraryPluginProvider is a PluginProvider that can load shared library plugins at runtime.

---

in the near future, combined with #2301, in ASTGen, we can easily create in-process "plugin server" like

@_spi(Compiler) import SwiftCompilerPluginMessageHandling

let inProcPlugins = CompilerPluginMessageHandler(provider: LibraryPluginProvier.shared)

// Load in-process plugins.
let response = inProcPlugins.handleMessage(
  HostToPluginMessage.loadPluginLibrary(libraryPath: path, moduleName: module)
)

// Expand macros.
let response = inProcPlugins.handleMessage(
  HostToPluginMessage.expandFreestandingMacro(...)
)

so we can treat in-process plugins as just another plugin server

[rintaro]

@swift-ci Please smoke test

[ahoppen]

Do we also need to support actors? Or are they mangled as classes?

[rintaro]

Apparently actors are mangled as classes.

[rintaro]

apple/swift#69236
@swift-ci Please test

[rintaro]

@swift-ci Please test

[rintaro]

@swift-ci Please test Windows

[rintaro]

@swift-ci Please test

[rintaro]

@swift-ci Please test Windows

[rintaro]

@swift-ci Please test Windows

[rintaro]

@swift-ci Please test

[rintaro]

@swift-ci Please test Windows

[rintaro]

@swift-ci Please test macOS

[rintaro]

Suggested change

	// NOTE: Do not use '_'
	// NOTE: Do not use '_SwiftSyntaxCShims' for 'dlopen' and 'LoadLibraryW' (Windows) because we don't want other modules depend on 'WinSDK'.

[rintaro]

@swift-ci Please test

[rintaro]

@swift-ci Please test Windows