Improvements to source file position tracking.

Be better about tracking the full extent of expressions in the parser;
this leads to more intelligible error messages when we indicate where
exactly the error happened.

module.cpp

@@ -275,7 +275,7 @@ Module::AddGlobalVariable(Symbol *sym, Expr *initExpr, bool isConst) {
                             dynamic_cast<ConstExpr *>(initExpr);
                 }
                 else
-                    Error(sym->pos, "Initializer for global variable \"%s\" "
+                    Error(initExpr->pos, "Initializer for global variable \"%s\" "
                           "must be a constant.", sym->name.c_str());
             }
         }