PR libstdc++/90634 reduce allocations in filesystem::path construction

PR libstdc++/90634 reduce allocations in filesystem::path construction

	PR libstdc++/90634
	* include/experimental/bits/fs_path.h (path::path(path&&)): Only call
	_M_split_cmpts() for a path with multiple components.
	(path::_S_is_dir_sep()): Add missing 'static' keyword to function.
	* src/filesystem/path.cc (path::_M_split_cmpts()): Count number of
	components and reserve space in vector. Return early when there is
	only one component.
	* testsuite/27_io/filesystem/path/construct/90634.cc: New test.
	* testsuite/experimental/filesystem/path/construct/90634.cc: New test.

From-SVN: r271717