[clang][ExprConst] Don't diagnose a non-existent init as not constant (#124575)

This test:
```c++
extern Swim& trident; // expected-note {{declared here}}

constexpr auto& gallagher = typeid(trident);    // expected-error {{constexpr variable 'gallagher' must be initialized by a constant expression}}
                                                // expected-note@-1 {{initializer of 'trident' is not a constant expression}}
```
diagnosed the initializer of `trident` as not constant, but `trident`
doesn't even have an initializer. Remove that diagnostic in this case.

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