[clang] [Clang] Diagnose forming references to nullptr (PR #143667)

[Shafik Yaghmour via cfe-commits]

================
@@ -1321,3 +1321,24 @@ constexpr bool check = different_in_loop();
   // expected-error at -1 {{}} expected-note at -1 {{in call}}
 
 }
+
+namespace GH48665 {
+constexpr bool foo(int *i) {
----------------
shafik wrote:

Other tests: https://godbolt.org/z/o66Gr3fc8

```cpp
struct A {
   int &r;
};

struct B {
  constexpr B(int *p) : r{*p} {
  }  
  int &r;
};

constexpr bool f(int *i) {
    int *p = new int;
    delete p;
    int &r = *p;

    
    A a{*i};
    B b(i);

    return true;
}

static_assert(f(nullptr), "");
```

https://github.com/llvm/llvm-project/pull/143667