Fix bugs in filesystem::path::lexically_normal()

Fix bugs in filesystem::path::lexically_normal()

Using path::_List::erase(const_iterator) to remove a non-final component
in path::lexically_normal() is a bug, because it leaves the following
component with an incorrect _M_pos value.

Instead of providing erase members that allow removing components from
the middle, replace them with pop_back() and
_M_erase_from(const_iterator) which only allow removing elements at the
end. Most uses of erase are unaffected, because they only remove
elements from the end anyway. The one use of erasure from the middle in
lexically_normal() is replaced by calls to pop_back() and/or clearing
the last component to leave it as an empty final filename.

Also replace the "???" comment in lexically_normal() to document when
that branch is taken.

	* include/bits/fs_path.h (path::_List::erase): Replace both overloads
	with ...
	(path::pop_back(), path::_M_erase_from(const_iterator)): New member
	functions that will only erase elements at the end.
	* src/filesystem/std-path.cc (path::_List::_Impl::pop_back()): Define.
	(path::_List::_Impl::_M_erase_from(const_iterator)): Define.
	(path::_List::operator=(const _List&)): Use _M_erase_from(p) instead
	of erase(p, end()).
	(path::_List::pop_back()): Define.
	(path::_List::_M_erase_from(const_iterator)): Define.
	(path::operator/=(const path&)): Use pop_back to remove last component
	and _M_erase_from to remove multiple components.
	(path::_M_append(basic_string_view<value_type>)): Likewise.
	(path::operator+=(const path&)): Likewise.
	(path::_M_concat(basic_string_view<value_type>)): Likewise.
	(path::remove_filename()): Likewise.
	(path::lexically_normal()): Use _List::_Impl iterators instead of
	path::iterator. Use pop_back to remove components from the end. Clear
	trailing filename, instead of using erase(const_iterator) to remove
	a non-final component.
	* testsuite/27_io/filesystem/path/generation/normal.cc: Test
	additional cases.
	* testsuite/27_io/filesystem/path/generation/normal2.cc: New test.

From-SVN: r267576