libstdc++: Add feature test macro for heterogeneous lookup in unordered containers

libstdc++: Add feature test macro for heterogeneous lookup in unordered containers

Also update the C++20 status docs.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* doc/xml/manual/status_cxx2020.xml:
	* doc/html/*: Regenerate.
	* include/bits/hashtable.h (__cpp_lib_generic_unordered_lookup):
	Define.
	* include/std/version (__cpp_lib_generic_unordered_lookup):
	Define.
	* testsuite/23_containers/unordered_map/operations/1.cc: Check
	feature test macro.
	* testsuite/23_containers/unordered_set/operations/1.cc:
	Likewise.