[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.

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -3600,8 +3600,12 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
        VD->mightBeUsableInConstantExpressions(Info.Ctx)) ||
       ((Info.getLangOpts().CPlusPlus || Info.getLangOpts().OpenCL) &&
        !Info.getLangOpts().CPlusPlus11 && !VD->hasICEInitializer(Info.Ctx))) {
-    Info.CCEDiag(E, diag::note_constexpr_var_init_non_constant, 1) << VD;
-    NoteLValueLocation(Info, Base);
+    if (Init) {
+      Info.CCEDiag(E, diag::note_constexpr_var_init_non_constant, 1) << VD;
+      NoteLValueLocation(Info, Base);
+    } else {
+      Info.CCEDiag(E);
+    }
   }
 
   // Never use the initializer of a weak variable, not even for constant

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -1462,7 +1462,7 @@ namespace InstantiateCaseStmt {
 
 namespace ConvertedConstantExpr {
   extern int &m;
-  extern int &n; // expected-note 2{{declared here}}
+  extern int &n; // pre-cxx23-note 2{{declared here}}
 
   constexpr int k = 4;
   int &m = const_cast<int&>(k);
@@ -1471,9 +1471,9 @@ namespace ConvertedConstantExpr {
   // useless note and instead just point to the non-constant subexpression.
   enum class E {
     em = m,
-    en = n, // cxx23-note {{initializer of 'n' is not a constant expression}} expected-error {{enumerator value is not a constant expression}} cxx11_20-note {{initializer of 'n' is unknown}}
-    eo = (m + // expected-error {{not a constant expression}}
-          n // cxx23-note {{initializer of 'n' is not a constant expression}} cxx11_20-note {{initializer of 'n' is unknown}}
+    en = n, // expected-error {{enumerator value is not a constant expression}} cxx11_20-note {{initializer of 'n' is unknown}}
+    eo = (m + // pre-cxx23-error {{not a constant expression}}
+          n // cxx11_20-note {{initializer of 'n' is unknown}} cxx23-error {{not a constant expression}}
           ),
     eq = reinterpret_cast<long>((int*)0) // expected-error {{not a constant expression}} expected-note {{reinterpret_cast}}
   };

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -47,11 +47,10 @@ void splash(Swim& swam) {
 }
 
 extern Swim dc;
-extern Swim& trident; // expected-note {{declared here}}
+extern Swim& trident;
 
 constexpr auto& sandeno   = typeid(dc);         // ok: can only be typeid(Swim)
 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}}
 
 namespace explicitThis {
 struct C {