[TBDGen] Consider -enable-testing for default arg generator symbols.

Fixes rdar://problem/40736382.

Author: huonw

lib/TBDGen/TBDGen.cpp

@@ -131,11 +131,15 @@ void TBDGenVisitor::visitAbstractFunctionDecl(AbstractFunctionDecl *AFD) {
     addSymbol(SILDeclRef(AFD).asForeign());
   }
 
-  if (!SwiftModule->getASTContext().isSwiftVersion3())
+  auto publicDefaultArgGenerators =
+      SwiftModule->getASTContext().isSwiftVersion3() ||
+      SwiftModule->isTestingEnabled();
+  if (!publicDefaultArgGenerators)
     return;
 
-  // In Swift 3, default arguments (of public functions) are public symbols,
-  // as the default values are computed at the call site.
+  // In Swift 3 (or under -enable-testing), default arguments (of public
+  // functions) are public symbols, as the default values are computed at the
+  // call site.
   auto index = 0;
   auto paramLists = AFD->getParameterLists();
   // Skip the first arguments, which contains Self (etc.), can't be defaulted,

test/TBD/function.swift

@@ -1,5 +1,7 @@
 // RUN: %target-swift-frontend -emit-ir -o- -parse-as-library -module-name test -validate-tbd-against-ir=all -swift-version 3 %s
 // RUN: %target-swift-frontend -emit-ir -o- -parse-as-library -module-name test -validate-tbd-against-ir=all -swift-version 4 %s
+// RUN: %target-swift-frontend -emit-ir -o- -parse-as-library -module-name test -validate-tbd-against-ir=all -swift-version 3 %s -enable-testing
+// RUN: %target-swift-frontend -emit-ir -o- -parse-as-library -module-name test -validate-tbd-against-ir=all -swift-version 4 %s -enable-testing
 
 public func publicNoArgs() {}
 public func publicSomeArgs(_: Int, x: Int) {}