[clang] [clang][ASTImporter] Fix possible crash "given incorrect InsertPos for specialization". (PR #89887)

[Qizhi Hu via cfe-commits]

jcsxky wrote:

> A test is needed to make the change acceptable but I could not find an easy case to provoke the situation. The problem looks to be related to his code:
> 
> ```c++
> using size_t = int;
> template<typename... _Types> class tuple;
> 
> template<class T, T v>
> struct integral_constant
> {
>     static constexpr T value = v;
>     using value_type = T;
>     using type = integral_constant; // using injected-class-name
>     constexpr operator value_type() const noexcept { return value; }
>     constexpr value_type operator()() const noexcept { return value; } // since c++14
> };
> 
> using true_type = integral_constant<bool, true>;
> using false_type = integral_constant<bool, false>;
> 
> template<class T, class U>
> struct is_same : false_type {};
>  
> template<class T>
> struct is_same<T, T> : true_type {};
> 
> template< class T, class U >
> inline constexpr bool is_same_v = is_same<T, U>::value;
> 
> namespace A {
>   template<typename _Tp, typename _Tuple>
>   struct __tuple_count;
> 
>   template<typename _Tp, typename _Tuple>
>     inline constexpr size_t __tuple_count_v =
>       __tuple_count<_Tp, _Tuple>::value;
> 
>   template<typename _Tp, typename... _Types>
>     struct __tuple_count<_Tp, tuple<_Types...>>
>     : integral_constant<size_t, 0> { };
> 
>   template<typename _Tp, typename _First, typename... _Rest>
>     struct __tuple_count<_Tp, tuple<_First, _Rest...>>
>     : integral_constant<
> 	size_t,
> 	__tuple_count_v<_Tp, tuple<_Rest...>> + is_same_v<_Tp, _First>> { };
> };
> 
> 
> size_t x = A::__tuple_count_v<int, tuple<bool, int, float>>;
> ```

Could you please show your commands which reproduced this crash? I tested locally with the following commands and it runs OK.
```cpp
clang++ -cc1 -std=c++17 -emit-pch -o test.cpp.ast test.cpp
clang++ -cc1 -x c++ -ast-merge test.cpp.ast  /dev/null -ast-dump
```

https://github.com/llvm/llvm-project/pull/89887