PR libstdc++/89117 fix path::replace_extension("") case

PR libstdc++/89117 fix path::replace_extension("") case

Previously the operator+=(extension) call would have re-parsed the path
and recreated the components with the right extension. Since optimising
it to not re-parse the whole string, we need to actually remove the
extension from the final filename before appending anything to it, and
append the dot to that final component too.

	PR libstdc++/89117
	* src/c++17/fs_path.cc (path::replace_extension): Erase extension from
	final component as well as from _M_pathname. Append the dot using
	operator+= instead of only to _M_pathname.
	(path::_M_find_extension): Reformat slightly.
	* testsuite/27_io/filesystem/path/modifiers/replace_extension.cc:
	Add more test cases.

From-SVN: r268406