Use more typical syntax

Author: H-G-Hristov

libcxx/include/istream

@@ -294,9 +294,10 @@ public:
   basic_istream& getline(char_type* __s, streamsize __n, char_type __dlm);
 
   basic_istream& ignore(streamsize __n = 1, int_type __dlm = traits_type::eof());
-  template <class _Tp = char_type>
-  inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 typename std::enable_if<is_same<_Tp, char>::value, basic_istream&>::type
-  ignore(streamsize __n, char_type __delim) {
+  // template <class _Tp = char_type,
+  //           class = typename enable_if<is_same<_Tp, char_type>::value>::type>
+  template <class _Tp = char_type, __enable_if_t<is_same<_Tp, char_type>::value, int> = 0>
+  inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 basic_istream& ignore(streamsize __n, char_type __delim) {
     return ignore(__n, traits_type::to_int_type(__delim));
   }
   int_type peek();