libstdc++: Fix incorrect preprocessor conditions in <version>

The conditions that guard the feature test macros in <version> should
match the main definitions of the macros in other headers.

This doesn't matter for GCC, because it supports all the conditions
being tested here, but it does matter for non-GCC compilers without the
relevant C++20 features.

libstdc++-v3/ChangeLog:

	* include/std/version (__cpp_lib_variant): Fix conditions to
	match <variant>.
	(__cpp_lib_expected): Fix condition to match <expected>.