[clang] [clang] Warn when builtin names are used outside of invocations (PR #96097)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

What if we went in the other direction? We want to deprecate `__is_pointer` as an identifier, so only make it an identifier when it is being used by libstdc++ as an identifier. libstdc++ usage looks something like:

```c++
// type template
template<typename T>
struct __is_pointer;
template<typename T>
struct __is_pointer<T*> : true_type {};
__is_pointer<T>::value

// variable template
template<typename T>
inline constexpr bool __is_pointer = ...
__is_pointer<T>
```

So only make it an identifier if it is preceded by `struct`/`class` or if it is succeeded by `<`/`=`?

https://github.com/llvm/llvm-project/pull/96097