Don't complain when using '@testable import' on a Clang-only module.

Justification: if you delete all the Swift code out of a framework,
you shouldn't have to change your tests.

rdar://problem/21207346

Swift SVN r29452

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -878,6 +878,8 @@ Module *ClangImporter::Implementation::finishLoadingClangModule(
     // but that's not correct for submodules.
     Identifier name = SwiftContext.getIdentifier((*clangModule).Name);
     result = Module::create(name, SwiftContext);
+    // Silence error messages about testably importing a Clang module.
+    result->setTestingEnabled();
 
     wrapperUnit =
       new (SwiftContext) ClangModuleUnit(*result, owner, clangModule);
@@ -1019,6 +1021,8 @@ ClangModuleUnit *ClangImporter::Implementation::getWrapperForModule(
   // FIXME: Handle hierarchical names better.
   Identifier name = SwiftContext.getIdentifier(underlying->Name);
   auto wrapper = Module::create(name, SwiftContext);
+  // Silence error messages about testably importing a Clang module.
+  wrapper->setTestingEnabled();
 
   auto file = new (SwiftContext) ClangModuleUnit(*wrapper, importer,
                                                  underlying);

test/attr/Inputs/custom-modules/module.map

@@ -1 +1,2 @@
 module AttrObjc_FooClangModule { header "attr_objc_foo_clang_module.h" }
+module Testable_ClangModule { header "testable_clang.h" }

test/attr/Inputs/custom-modules/testable_clang.h

@@ -0,0 +1 @@
+extern int clangGlobal;

test/attr/testable.swift

@@ -1,10 +1,13 @@
 // RUN: rm -rf %t && mkdir %t
 // RUN: %target-swift-frontend -emit-module -enable-testing -o %t %S/../Inputs/empty.swift
-// RUN: %target-swift-frontend -parse -I %t %s -verify
-// RUN: %target-swift-frontend -parse -I %t %s -disable-testable-attr-requires-testable-module -DIMPORTS_ONLY
+// RUN: %target-swift-frontend -parse -I %t -I %S/Inputs/custom-modules %s -verify
+// RUN: %target-swift-frontend -parse -I %t -I %S/Inputs/custom-modules %s -disable-testable-attr-requires-testable-module -DIMPORTS_ONLY
 
 @testable import Swift // expected-error {{module 'Swift' was not compiled for testing}}
 @testable import empty // no-error
+@testable import Testable_ClangModule // no-error
+
+_ = clangGlobal
 
 #if !IMPORTS_ONLY
 @testable func foo() {} // expected-error {{@testable may only be used on 'import' declarations}}