AST: ASTPrinter support for 'reasync'

Part of <rdar://problem/71098795>.

Author: slavapestov

lib/AST/ASTPrinter.cpp

@@ -3152,7 +3152,10 @@ void PrintAST::printFunctionParameters(AbstractFunctionDecl *AFD) {
 
   if (AFD->hasAsync()) {
     Printer << " ";
-    Printer.printKeyword("async", Options);
+    if (AFD->getAttrs().hasAttribute<ReasyncAttr>())
+      Printer.printKeyword("reasync", Options);
+    else
+      Printer.printKeyword("async", Options);
   }
 
   if (AFD->hasThrows()) {

lib/AST/Attr.cpp

@@ -724,6 +724,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
   case DAK_ObjCBridged:
   case DAK_SynthesizedProtocol:
   case DAK_Rethrows:
+  case DAK_Reasync:
   case DAK_Infix:
     return false;
   case DAK_Override: {

test/ModuleInterface/concurrency.swift

@@ -8,6 +8,10 @@ public func fn() async {
   fatalError()
 }
 
+public func reasyncFn(_: () async -> ()) reasync {
+  fatalError()
+}
+
 // RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -I %t
 
 #else
@@ -21,3 +25,4 @@ func callFn() async {
 // RUN: %FileCheck %s <%t/Library.swiftinterface
 // CHECK: // swift-module-flags:{{.*}} -enable-experimental-concurrency
 // CHECK: public func fn() async
+// CHECK: public func reasyncFn(_: () async -> ()) reasync