[clang] [clang] Track source deduction guide for alias template deduction guides (PR #123875)

[via cfe-commits]

================
@@ -8147,6 +8149,26 @@ TEST_P(ImportFunctions, CTADWithLocalTypedef) {
   ASSERT_TRUE(ToD);
 }
 
+TEST_P(ImportFunctions, CTADAliasTemplate) {
+  Decl *TU = getTuDecl(
+      R"(
+      template <typename T> struct A {
+        A(T);
+      };
+      template<typename T>
+      using B = A<T>;
+      B b{(int)0};
+      )",
+      Lang_CXX20, "input.cc");
+  auto *FromD = FirstDeclMatcher<CXXDeductionGuideDecl>().match(
+      TU, cxxDeductionGuideDecl(hasParameter(0, hasType(asString("int")))));
+  auto *ToD = Import(FromD, Lang_CXX20);
+  ASSERT_TRUE(ToD);
+  EXPECT_TRUE(ToD->getSourceDeductionGuideKind() ==
+              CXXDeductionGuideDecl::SourceDeductionGuideKind::Alias);
----------------
antangelo wrote:

Done, I was already testing `getSourceDeductionGuide` for the `nulltptr` case so I have now added assertions for `getSourceDeductionGuideKind` being `None`.

https://github.com/llvm/llvm-project/pull/123875