PR libstdc++/50862
* include/std/condition_variable (condition_variable_any::wait): Avoid
terminating if relocking user mutex throws during stack-unwinding.
* testsuite/30_threads/condition_variable_any/50862.cc: Add dg-require.
Fixes two more issues pointed out in
PR libstdc++/50862
* include/std/condition_variable (condition_variable_any::wait): Fix
deadlock and ensure _Lock::lock() is called on exit.
(condition_variable_any::native_handle): Remove, as per LWG 1500.
* testsuite/30_threads/condition_variable_any/50862.
