Fix check for main function already declared

src/context.rs

@@ -526,7 +526,7 @@ impl<'gcc, 'tcx> MiscMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
 
     fn declare_c_main(&self, fn_type: Self::Type) -> Option<Self::Function> {
         let entry_name = self.sess().target.entry_name.as_ref();
-        if self.get_declared_value(entry_name).is_none() {
+        if !self.functions.borrow().contains_key(entry_name) {
             Some(self.declare_entry_fn(entry_name, fn_type, ()))
         } else {
             // If the symbol already exists, it is an error: for example, the user wrote