[Macros] Add a plugin capability for infer-nonisolated-conformances

The compiler will need to be able to distinguish between macro
implementations that know about infer-nonisolated-conformances (such
that the plugin itself can apply the transform) vs. those that predate
this transform (in which case the compiler will need to do it).

Author: DougGregor

Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift

@@ -30,6 +30,9 @@ import SwiftSyntaxMacros
 @_spi(PluginMessage)
 public enum PluginFeature: String {
   case loadPluginLibrary = "load-plugin-library"
+
+  /// Whether the plugin knows how to infer nonisolated conformances.
+  case inferNonisolatedConformances = "infer-nonisolated-conformances"
 }
 
 /// A type that provides the actual plugin functions.

Sources/SwiftLibraryPluginProvider/LibraryPluginProvider.swift

@@ -84,7 +84,7 @@ public class LibraryPluginProvider: PluginProvider {
   public static let shared: LibraryPluginProvider = LibraryPluginProvider()
 
   public var features: [PluginFeature] {
-    [.loadPluginLibrary]
+    [.loadPluginLibrary, .inferNonisolatedConformances]
   }
 
   public func loadPluginLibrary(libraryPath: String, moduleName: String) throws {