PR libstdc++/49894
* include/std/mutex (__mutex_base,__recursive_mutex_base): Define new
base classes to manage construction/destruction of native mutexes,
using NSDMI when INIT macros are defined.
(mutex,recursive_mutex,timed_mutex,recursive_timed_mutex): Derive from
new base classes.
* include/std/condition_variable (condition_variable): Use NSDMI when
INIT macro is defined. Use noexcept.
* src/condition_variable.cc (condition_variable): Explicitly-default
constructor/destructor when using NSDMI. Use noexcept.
(condition_variable_any): Likewise.
From-SVN: r180411
