This doesn't actually have any effect unless you also change the
predefined value of __cplusplus, as it's currently 201703L. But if
somebody does want to do that, the new headers will get processed now.
libstdc++-v3/ChangeLog:
	* doc/doxygen/user.cfg.in (INPUT): Add <latch> and <semaphore>.
                          include/iostream \
                          include/istream \
                          include/iterator \
+                         include/latch \
                          include/limits \
                          include/list \
                          include/locale \
                          include/ratio \
                          include/regex \
                          include/scoped_allocator \
+                         include/semaphore \
                          include/set \
                          include/shared_mutex \
                          include/span \