[SE-0289] Continue printing @_functionBuilder in .swiftinterfaces.

Maintain the ability for older Swift compilers to read .swiftinterfaces
that make use of result builders by always emitting @_functionBuilder
rather than the newer @resultBuilder.

Author: DougGregor

lib/AST/Attr.cpp

@@ -771,6 +771,10 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
   case DAK_Optimize:
     if (DeclAttribute::isDeclModifier(getKind())) {
       Printer.printKeyword(getAttrName(), Options);
+    } else if (Options.IsForSwiftInterface && getKind() == DAK_ResultBuilder) {
+      // Use @_functionBuilder in Swift interfaces to maintain backward
+      // compatibility.
+      Printer.printSimpleAttr("_functionBuilder", /*needAt=*/true);
     } else {
       Printer.printSimpleAttr(getAttrName(), /*needAt=*/true);
     }

test/ModuleInterface/Inputs/result_builders_client.swift

@@ -1,4 +1,4 @@
-import FunctionBuilders
+import ResultBuilders
 
 let name = "dsl"
 tuplify(true) {

test/ModuleInterface/result_builders.swift

@@ -1,9 +1,11 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -typecheck -module-name FunctionBuilders -emit-module-interface-path %t/FunctionBuilders.swiftinterface %s
-// RUN: %FileCheck %s < %t/FunctionBuilders.swiftinterface
+// RUN: %target-swift-frontend -typecheck -module-name ResultBuilders -emit-module-interface-path %t/ResultBuilders.swiftinterface %s
+// RUN: %FileCheck %s < %t/ResultBuilders.swiftinterface
 // RUN: %target-swift-frontend -I %t -typecheck -verify %S/Inputs/result_builders_client.swift
-// RUN: %target-swift-frontend -compile-module-from-interface %t/FunctionBuilders.swiftinterface -o %t/FunctionBuilders.swiftmodule
+// RUN: %target-swift-frontend -compile-module-from-interface %t/ResultBuilders.swiftinterface -o %t/ResultBuilders.swiftmodule
+// RUN: %FileCheck %s < %t/ResultBuilders.swiftinterface
 
+// CHECK: @_functionBuilder public struct TupleBuilder
 @resultBuilder
 public struct TupleBuilder {
   public static func buildBlock<T1, T2>(_ t1: T1, _ t2: T2) -> (T1, T2) {
@@ -38,7 +40,7 @@ public struct TupleBuilder {
   public static func buildIf<T>(_ value: T?) -> T? { return value }
 }
 
-// CHECK-LABEL: public func tuplify<T>(_ cond: Swift.Bool, @FunctionBuilders.TupleBuilder body: (Swift.Bool) -> T)
+// CHECK-LABEL: public func tuplify<T>(_ cond: Swift.Bool, @ResultBuilders.TupleBuilder body: (Swift.Bool) -> T)
 public func tuplify<T>(_ cond: Bool, @TupleBuilder body: (Bool) -> T) {
   print(body(cond))
 }
@@ -52,13 +54,13 @@ public struct UsesBuilderProperty {
     "goodbye"
   }
 
-  // CHECK: public func myFunc(@FunctionBuilders.TupleBuilder fn: () -> ())
+  // CHECK: public func myFunc(@ResultBuilders.TupleBuilder fn: () -> ())
   public func myFunc(@TupleBuilder fn: () -> ()) {}
 }
 
 public protocol ProtocolWithBuilderProperty {
   associatedtype Assoc
-  
-  // CHECK: @FunctionBuilders.TupleBuilder var myVar: Self.Assoc { get }
+
+  // CHECK: @ResultBuilders.TupleBuilder var myVar: Self.Assoc { get }
   @TupleBuilder var myVar: Assoc { get }
 }