[clang2ir] Fix translation of negative integral constants

and remove useless declaration

ClangVisitor.cpp

@@ -9169,8 +9169,7 @@ bool FramacVisitor::VisitVarDecl(clang::VarDecl* Decl) {
         clang::APValue* resultingValue = Decl->evaluateValue();
         if (resultingValue) {
           constantType = Decl->hasExternalStorage()?ICEXTERNCONST:ICSTATICCONST;
-          clang::APValue* resultingValue = Decl->evaluateValue();
-          value=(int64_t) resultingValue->getInt().getLimitedValue(UINT64_MAX);
+          value = resultingValue->getInt().getExtValue();
         }
       };
     };

tests/basic/oracle/init.res.oracle

@@ -46,8 +46,8 @@ struct _frama_c_rtti_name_info_node _frama_c_rtti_name_info =
    .number_of_base_classes = 0,
    .pvmt = (struct _frama_c_vmt *)0};
 myStruct s = {.a = {(char)0}};
-static int const I = (int)4294967295;
-static long const L = (long)4294967295;
+static int const I = -1;
+static long const L = (long)(-1);
 static long long const LL = (long long)(-1);
 int main(void)
 {