[clang][Interp] Don't suppress diagnostics for undefined+external funcs

Calling them should still generate a diagnostic.

Author: tbaederr

clang/lib/AST/Interp/Interp.cpp

@@ -485,7 +485,9 @@ bool CheckCallable(InterpState &S, CodePtr OpPC, const Function *F) {
         // Don't emit anything if the function isn't defined and we're checking
         // for a constant expression. It might be defined at the point we're
         // actually calling it.
-        if (!DiagDecl->isDefined() && S.checkingPotentialConstantExpression())
+        bool IsExtern = DiagDecl->getStorageClass() == SC_Extern;
+        if (!DiagDecl->isDefined() && !IsExtern &&
+            S.checkingPotentialConstantExpression())
           return false;
 
         // If the declaration is defined _and_ declared 'constexpr', the below

clang/test/AST/Interp/records.cpp

@@ -1238,3 +1238,9 @@ namespace InvalidCtorInitializer {
   // no crash on evaluating the constexpr ctor.
   constexpr int Z = X().Y; // both-error {{constexpr variable 'Z' must be initialized by a constant expression}}
 }
+
+extern int f(); // both-note {{here}}
+struct HasNonConstExprMemInit {
+  int x = f(); // both-note {{non-constexpr function}}
+  constexpr HasNonConstExprMemInit() {} // both-error {{never produces a constant expression}}
+};