Add feature for _implicitSelfCapture attribute. (swiftlang#37335)

Fixes rdar://77681413.

Author: DougGregor

include/swift/Basic/Features.def

@@ -48,5 +48,6 @@ LANGUAGE_FEATURE(BuiltinExecutor, 0, "Executor builtins", true)
 LANGUAGE_FEATURE(BuiltinContinuation, 0, "Continuation builtins", true)
 LANGUAGE_FEATURE(BuiltinTaskGroup, 0, "TaskGroup builtins", true)
 LANGUAGE_FEATURE(InheritActorContext, 0, "@_inheritActorContext attribute", true)
+LANGUAGE_FEATURE(ImplicitSelfCapture, 0, "@_implicitSelfCapture attribute", true)
 
 #undef LANGUAGE_FEATURE

lib/AST/ASTPrinter.cpp

@@ -2799,6 +2799,17 @@ static bool usesFeatureInheritActorContext(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureImplicitSelfCapture(Decl *decl) {
+  if (auto func = dyn_cast<AbstractFunctionDecl>(decl)) {
+    for (auto param : *func->getParameters()) {
+      if (param->getAttrs().hasAttribute<ImplicitSelfCaptureAttr>())
+        return true;
+    }
+  }
+
+  return false;
+}
+
 /// Determine the set of "new" features used on a given declaration.
 ///
 /// Note: right now, all features we check for are "new". At some point, we'll