[flang] Update printing values in dump-parse-tree

Remove 'if std::string' that is covered by another branch of the
if-statement.
Add printing of 'bool' and 'int' values, since they have corresponding
`GetNodeName` definitions.

Author: kparzysz

flang/include/flang/Parser/dump-parse-tree.h

@@ -884,8 +884,10 @@ class ParseTreeDumper {
     } else if constexpr (HasSource<T>::value) {
       return x.source.ToString();
 #endif
-    } else if constexpr (std::is_same_v<T, std::string>) {
-      return x;
+    } else if constexpr (std::is_same_v<T, int>) {
+      return std::to_string(x);
+    } else if constexpr (std::is_same_v<T, bool>) {
+      return x ? "true" : "false";
     } else {
       return "";
     }