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.