[clang] [Clang][Sema] Handle invalid variable template specialization whose type depends on itself (PR #134522)

[Erich Keane via cfe-commits]

================
@@ -47,3 +47,21 @@ namespace InvalidInsertPos {
   template<> int v<int, 0>;
   int k = v<int, 500>;
 }
+
+namespace GH51347 {
+  template <typename T>
+  auto p = p<T>; // expected-error {{the type of variable template specialization 'p<int>'}}
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erichkeane wrote:

It should be the same/similar as when we recognize the `auto x= x +1 ` or `int x = x + 1;` (one is an error, the other is a warning).

This is probably fine if this isn't a primary template, right?  

Generally, Clang templates try to diagnose things as absolute early as possible so we get ONE error instead of one-per-instantiation.  

That said, it is perhaps worth spending some time to see if there is a reason we can't, or, by language rule, shouldn't diagnose this on `auto p = p <T>;`

https://github.com/llvm/llvm-project/pull/134522