Add Builtin.hopToActor to the feature list.

This builtin never occurs in @inlinable code. But apparently we still
need to add a language feature for every builtin. This must allow
older compilers to reparse the library source (though I don't know why
that would ever happen!)

Fixes rdar://80525569 error: module 'Builtin' has no member named 'hopToActor')

Author: atrick

include/swift/Basic/Features.def

@@ -47,6 +47,7 @@ LANGUAGE_FEATURE(BuiltinJob, 0, "Builtin.Job type", true)
 LANGUAGE_FEATURE(Sendable, 0, "Sendable and @Sendable", true)
 LANGUAGE_FEATURE(BuiltinExecutor, 0, "Builtin.Executor type", true)
 LANGUAGE_FEATURE(BuiltinContinuation, 0, "Continuation builtins", true)
+LANGUAGE_FEATURE(BuiltinHopToActor, 0, "Builtin.HopToActor", true)
 LANGUAGE_FEATURE(BuiltinTaskGroupWithArgument, 0, "TaskGroup builtins", true)
 LANGUAGE_FEATURE(InheritActorContext, 0, "@_inheritActorContext attribute", true)
 LANGUAGE_FEATURE(ImplicitSelfCapture, 0, "@_implicitSelfCapture attribute", true)

lib/AST/ASTPrinter.cpp

@@ -2713,6 +2713,10 @@ static bool usesFeatureBuiltinContinuation(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureBuiltinHopToActor(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureBuiltinTaskGroupWithArgument(Decl *decl) {
   return false;
 }

stdlib/public/Concurrency/Task.swift

@@ -762,7 +762,11 @@ public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
   Task.detached {
     do {
 #if !os(Windows)
+#if compiler(>=5.5) && $BuiltinHopToActor
       Builtin.hopToActor(MainActor.shared)
+#else
+      fatalError("Swift compiler is incompatible with this SDK version")
+#endif
 #endif
       try await asyncFun()
       exit(0)