When recursing into a directory, any errors that occur while removing a
directory entry are ignored, because the subsequent increment of the
directory iterator clears the error_code object.

This fixes that bug by checking the result of each recursive operation
before incrementing. This is a change in observable behaviour, because
previously other directory entries would still be removed even if one
(or more) couldn't be removed due to errors. Now the operation stops on
the first error, which is what the code intended to do all along. The
standard doesn't specify what happens in this case (because the order
that the entries are processed is unspecified anyway).

It also improves the error reporting so that the name of the file that
could not be removed is included in the filesystem_error exception. This
is done by introducing a new helper type for reporting errors with
additional context and a new function that uses that type. Then the
overload of std::filesystem::remove_all that throws an exception can use
the new function to ensure any exception contains the additional
information.

For std::experimental::filesystem::remove_all just fix the bug where
errors are ignored.

	PR libstdc++/93201
	* src/c++17/fs_ops.cc (do_remove_all): New function implementing more
	detailed error reporting for remove_all. Check result of recursive
	call before incrementing iterator.
	(remove_all(const path&), remove_all(const path&, error_code&)): Use
	do_remove_all.
	* src/filesystem/ops.cc (remove_all(const path&, error_code&)): Check
	result of recursive call before incrementing iterator.
	* testsuite/27_io/filesystem/operations/remove_all.cc: Check errors
	are reported correctly.
	* testsuite/experimental/filesystem/operations/remove_all.cc: Likewise.

From-SVN: r280014