libstdc++: Do not define _Insert_base::try_emplace before C++17

This is not a reserved name in C++11 and C++14, so must not be defined.

Also use the appropriate feature test macros for the try_emplace members
of the Debug Mode maps.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Insert_base::try_emplace):
	Do not define for C++11 and C++14.
	* include/debug/map.h (try_emplace): Use feature test macro.
	* include/debug/unordered_map (try_emplace): Likewise.
	* testsuite/17_intro/names.cc: Define try_emplace before C++17.

libstdc++-v3/include/bits/hashtable_policy.h

@@ -1008,6 +1008,7 @@ namespace __detail
 	return __h._M_insert(__hint, __v, __node_gen, __unique_keys{});
       }
 
+#ifdef __glibcxx_unordered_map_try_emplace // C++ >= 17 && HOSTED
       template<typename _KType, typename... _Args>
 	std::pair<iterator, bool>
 	try_emplace(const_iterator, _KType&& __k, _Args&&... __args)
@@ -1029,6 +1030,7 @@ namespace __detail
 	  __node._M_node = nullptr;
 	  return { __it, true };
 	}
+#endif
 
       void
       insert(initializer_list<value_type> __l)

libstdc++-v3/include/debug/map.h

@@ -344,7 +344,7 @@ namespace __debug
 	}
 
 
-#if __cplusplus > 201402L
+#ifdef __glibcxx_map_try_emplace // C++ >= 17 && HOSTED
       template <typename... _Args>
         pair<iterator, bool>
         try_emplace(const key_type& __k, _Args&&... __args)

libstdc++-v3/include/debug/unordered_map

@@ -442,7 +442,7 @@ namespace __debug
 	  _M_check_rehashed(__bucket_count);
 	}
 
-#if __cplusplus > 201402L
+#ifdef __glibcxx_unordered_map_try_emplace // C++ >= 17 && HOSTED
       template <typename... _Args>
 	pair<iterator, bool>
 	try_emplace(const key_type& __k, _Args&&... __args)

libstdc++-v3/testsuite/17_intro/names.cc

@@ -138,6 +138,8 @@
 // <charconv> defines to_chars_result::ptr and to_chars_result::ec
 #define ec (
 #define ptr (
+// <map> and <unordered_map> define try_emplace
+#define try_emplace (
 #endif
 
 // These clash with newlib so don't use them.