Define the feature test macro now that ranges support is complete.

This also changes the preprocessor checks for the __cpp_concepts macro
so that library components depending on concepts are only enabled when
C++20 concepts are supported, and not just for the Concepts TS (which
uses different syntax in places).

	* include/bits/range_cmp.h (__cpp_lib_ranges): Define.
	* include/bits/stl_iterator.h: Check value of __cpp_concepts so that
	C++20 concepts are required.
	* include/bits/stl_iterator_base_types.h: Likewise.
	* include/std/concepts: Likewise.
	* include/std/version: Likewise.
	* testsuite/std/ranges/headers/ranges/synopsis.cc: Check feature test
	macro.