[ASTImporter] Add unittest case for friend decl import

[vabridgers]

Summary

This change adds a matching test case for the recent bug fix to
VisitFriendDecl in ASTImporterLookup.cpp.

See https://reviews.llvm.org/D82882 for details.

[vabridgers]

Updated commit header.

[vabridgers]

update for pre-merge lint checks

[vabridgers]

I confirmed that a crash was seen when the change for https://reviews.llvm.org/D82882 was reverted, then the unittests pass with the change associated with https://reviews.llvm.org/D82882.

[martong]

Thanks, looks good!

clang/unittests/AST/ASTImporterTest.cpp

TEST_P(ASTImporterOptionSpecificTestBase,
   auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
   ASSERT_TRUE(ImportedSpec);
   auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
       ToTU, classTemplateSpecializationDecl(hasName("X")));
   EXPECT_EQ(ImportedSpec, ToSpec);
   EXPECT_TRUE(ToCtor->hasBody());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, ClassTemplateFriendDecl) {
+  const auto *Code =
+      R"(
+      template <class T> class X {  friend T; };
+      struct Y {};
+      template class X<Y>;
+    )";
+  Decl *ToTU = getToTuDecl(Code, Lang_CXX11);
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl());
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                    ToTU, classTemplateSpecializationDecl()));
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase,
        ClassTemplatePartialSpecializationsShouldNotBeDuplicated) {
   auto Code =
       R"(
     // primary template
     template<class T1, class T2, int I>
     class A {};