[clang] [clang][bytecode] Don't diagnose defined functions that will have a body (PR #165002)

[Timm Baeder via cfe-commits]

tbaederr wrote:

This breaks `SemaTemplate/instantiate-self.cpp`, in particular
```c++

  template<typename T> constexpr T f(T);
  template<typename T> constexpr T g(T t) {
    // FIXME: It'd be nice to say that the function is currently being defined, rather than being undefined.
    typedef int arr[f(T())]; // expected-error {{variable length array}} expected-note {{undefined function 'f<int>'}}
    return t;
  }
  template<typename T> constexpr T f(T t) { // expected-note {{declared here}}
    typedef int arr[g(T())]; // expected-error {{zero size array}} expected-note {{instantiation of}}
    return t;
  }
  int n = f(0); // expected-note 2{{instantiation of}}
```

https://github.com/llvm/llvm-project/pull/165002