libstdc++: Implement P4206R0: Revert string support in std::constant_wrapper.

libstdc++: Implement P4206R0: Revert string support in std::constant_wrapper.

We need to apply remove_cvref_t on decltype(_Xv) for default template argument
due PR115314. The constant_wrapper::value is declared as decltype((__Xv)) due
PR125188.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (constant_wrapper): Bump to 202606L.
	* include/bits/version.h: Regenerate.
	* include/bits/funcref_impl.h (function_ref::function_ref): Rename
	template parameter from __cwfn to __fn and use it direclty.
	* include/bits/funcwrap.h (function_ref): Rename template parameter
	to __fn.
	(std::constant_wrapper): Use auto as non-type template parameter,
	and refeference it as value.
	* include/bits/utility.h (__CwFixedValue): Remove.
	* testsuite/20_util/constant_wrapper/generic.cc: Remove arrays
	and string literal tests. Add test for address of value.
	* testsuite/20_util/constant_wrapper/other_wrappers.cc:
	Remove test_array.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>