libstdc++: Simplify resource management in directory iterators

This replaces the _Dir constructor that takes ownership of an existing
DIR* resource with one that takes a _Dir_base rvalue instead. This means
a raw DIR* is never passed around, but is always owned by a _Dir_base
object.

libstdc++-v3/ChangeLog:

	* src/c++17/fs_dir.cc (_Dir(DIR*, const path&)): Change first
	parameter to _Dir_base&&.
	* src/filesystem/dir-common.h (_Dir_base(DIR*)): Remove.
	* src/filesystem/dir.cc (_Dir(DIR*, const path&)): Change first
	parameter to _Dir_base&&.