libstdc++: Fix error handling in fs::hard_link_count for Windows

The recent change to use auto_win_file_handle for
std::filesystem::hard_link_count caused a regression. The
std::error_code argument should be cleared if no error occurs, but this
no longer happens. Add a call to ec.clear() in fs::hard_link_count to
fix this.

Also change the auto_win_file_handle class to take a reference to the
std::error_code and set it if an error occurs, to slightly simplify the
control flow in the fs::equiv_files function.

libstdc++-v3/ChangeLog:

	* src/c++17/fs_ops.cc (auto_win_file_handle): Add error_code&
	member and set it if CreateFileW or GetFileInformationByHandle
	fails.
	(fs::equiv_files) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Simplify
	control flow.
	(fs::hard_link_count) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Clear ec
	on success.
	* testsuite/27_io/filesystem/operations/hard_link_count.cc:
	Check error handling.

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