libstdc++: More user-friendly failed assertions from shared_ptr dereference

libstdc++: More user-friendly failed assertions from shared_ptr dereference

Currently dereferencing an empty shared_ptr prints a complicated
internal type in the assertion message:

include/bits/shared_ptr_base.h:1377: std::__shared_ptr_access<_Tp, _Lp, <anonymous>, <anonymous> >::element_type& std::__shared_ptr_access<_Tp, _Lp, <anonymous>, <anonymous> >::operator*() const [with _Tp = std::filesystem::__cxx11::recursive_directory_iterator::_Dir_stack; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic; bool <anonymous> = false; bool <anonymous> = false; element_type = std::filesystem::__cxx11::recursive_directory_iterator::_Dir_stack]: Assertion '_M_get() != nullptr' failed.

Users don't care about any of the _Lp and <anonymous> template
parameters, so this is unnecessarily verbose.

We can simplify it to something that only mentions "shared_ptr_deref"
and the element type:

include/bits/shared_ptr_base.h:1371: _Tp* std::__shared_ptr_deref(_Tp*) [with _Tp = filesystem::__cxx11::recursive_directory_iterator::_Dir_stack]: Assertion '__p != nullptr' failed.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_base.h (__shared_ptr_deref): New
	function template.
	(__shared_ptr_access, __shared_ptr_access<>): Use it.