[clang] [clang][ASTImporter] skip TemplateTypeParmDecl in VisitTypeAliasTemplateDecl (PR #74919)

[Balázs Kéri via cfe-commits]

balazske wrote:

The `VisitTypeAliasTemplateDecl` function should be re-designed to check for structural equivalence at import. The following test does not pass because an existing `TypeAliasTemplateDecl` declaration with the same name is always found and returned, without check for structural equivalence.
```
TEST_P(ASTImporterOptionSpecificTestBase, ImportTypeAliasTemplateDecl1) {
  const char *ToCode =
      R"(
      struct S;
      template <typename, typename>
      using Callable = S;
      )";
  const char *Code =
      R"(
      struct S;
      template <typename>
      using Callable = S;
      )";
  Decl *ToTU = getToTuDecl(ToCode, Lang_CXX17);
  Decl *FromTU = getTuDecl(Code, Lang_CXX17);

  auto *FromCallable = FirstDeclMatcher<TypeAliasTemplateDecl>().match(
      FromTU, typeAliasTemplateDecl(hasName("Callable")));

  auto *ToCallable = FirstDeclMatcher<TypeAliasTemplateDecl>().match(
      ToTU, typeAliasTemplateDecl(hasName("Callable")));

  auto *ImportedCallable = Import(FromCallable, Lang_CXX17);
  EXPECT_TRUE(ImportedCallable);
  EXPECT_NE(ImportedCallable, ToCallable);
}
```

Additionally I discovered that import of `ClassTemplateDecl` is not correct too: If there is an object with the same name that is not a `ClassTemplateDecl`, it is just ignored at import. This is not correct, the existing object may cause name conflict (for example it can be a non-template `RecordDecl`). (I found this when checking the questions in my last comment.) This is an independent problem but should be fixed.

https://github.com/llvm/llvm-project/pull/74919