[clang][ASTImporter] Add import of attribute 'enable_if'.

[martong]

LGTM

clang/lib/AST/ASTImporter.cpp

Expected<Attr *> ASTImporter::Import(const Attr *FromAttr) {
 
   case attr::Format: {
     const auto *From = cast<FormatAttr>(FromAttr);
     AI.importAttr(From, Import(From->getType()), From->getFormatIdx(),
                   From->getFirstArg());
     break;
   }
 
+  case attr::EnableIf: {
+    const auto *From = cast<EnableIfAttr>(FromAttr);
+    AI.importAttr(From, AI.importArg(From->getCond()).value(),
+                  From->getMessage());
+    break;
+  }
+
   case attr::AssertCapability: {
     const auto *From = cast<AssertCapabilityAttr>(FromAttr);
     AI.importAttr(From,
                   AI.importArrayArg(From->args(), From->args_size()).value(),
                   From->args_size());
     break;
   }
   case attr::AcquireCapability: {

clang/unittests/AST/ASTImporterTest.cpp

TEST_P(ASTImporterOptionSpecificTestBase, ImportFormatAttr) {
   EXPECT_EQ(FromAttr->isPackExpansion(), ToAttr->isPackExpansion());
   EXPECT_EQ(FromAttr->isImplicit(), ToAttr->isImplicit());
   EXPECT_EQ(FromAttr->getSyntax(), ToAttr->getSyntax());
   EXPECT_EQ(FromAttr->getAttributeSpellingListIndex(),
             ToAttr->getAttributeSpellingListIndex());
   EXPECT_EQ(FromAttr->getType()->getName(), ToAttr->getType()->getName());
 }
 
+TEST_P(ImportAttributes, ImportEnableIf) {
+  EnableIfAttr *FromAttr, *ToAttr;
+  importAttr<FunctionDecl>(
+      "void test(int A) __attribute__((enable_if(A == 1, \"message\")));",
+      FromAttr, ToAttr);
+  checkImported(FromAttr->getCond(), ToAttr->getCond());
+  EXPECT_EQ(FromAttr->getMessage(), ToAttr->getMessage());
+}
+
 TEST_P(ImportAttributes, ImportGuardedVar) {
   GuardedVarAttr *FromAttr, *ToAttr;
   importAttr<VarDecl>("int test __attribute__((guarded_var));", FromAttr,
                       ToAttr);
 }
 
 TEST_P(ImportAttributes, ImportPtGuardedVar) {
   PtGuardedVarAttr *FromAttr, *ToAttr;