libstdc++: Check feature test macro for std::string_view in <string>

We can use the __glibcxx_string_view macro to guard the uses of
std::string_view in <string>, instead of just checking the value of
__cplusplus. It makes no practical difference because
__glibcxx_string_view is defined for C++17 and up, but it makes it clear
to readers that the lines guarded by that macro are features that depend
on string_view.

We could be more precise and check __glibcxx_string_view >= 201606L
which is the value for the P0254R2 paper that integrated
std::string_view with std::string, but I think just checking for the
macro being defined is clear enough.

We can also check __glibcxx_variant for the _Never_valueless_alt partial
specialization.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: Check __glibcxx_string_view and
	__glibcxx_variant instead of __cplusplus >= 2017L.
	* include/bits/cow_string.h: Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

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