[clang][bytecode] Fix reporting non-constant variables in C (#109516)

We need to call FFDiag() to get the usual "invalid subexpression"
diagnostic.

clang/lib/AST/ByteCode/Interp.cpp

@@ -90,10 +90,12 @@ static bool diagnoseUnknownDecl(InterpState &S, CodePtr OpPC,
 
 static void diagnoseNonConstVariable(InterpState &S, CodePtr OpPC,
                                      const ValueDecl *VD) {
-  if (!S.getLangOpts().CPlusPlus)
-    return;
-
   const SourceInfo &Loc = S.Current->getSource(OpPC);
+  if (!S.getLangOpts().CPlusPlus) {
+    S.FFDiag(Loc);
+    return;
+  }
+
   if (const auto *VarD = dyn_cast<VarDecl>(VD);
       VarD && VarD->getType().isConstQualified() &&
       !VarD->getAnyInitializer()) {

clang/test/Sema/annotate-type.c

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify
+// RUN: %clang_cc1 %s -fsyntax-only -verify -fexperimental-new-constant-interpreter
 
 const char *some_function();