libstdc++: Implement P1972R2 changes to std::variant (PR 95832)

libstdc++: Implement P1972R2 changes to std::variant (PR 95832)

G++ implements P1972R2 since r11-1597-0ca22d027ecc and so we no longer
need the P0608R3 special case to prevent narrowing conversions to bool.

Since non-GNU compilers don't necessarily implment P1972R2 yet, this
may cause a regression for those compilers. There is no feature-test
macro we can use to detect it though, so we'll have to live with it.

libstdc++-v3/ChangeLog:

	PR libstdc++/95832
	* include/std/variant (__detail::__variant::_Build_FUN): Remove
	partial specialization to prevent narrowing conversions to bool.
	* testsuite/20_util/variant/compile.cc: Test non-narrowing
	conversions to bool.
	* testsuite/20_util/variant/run.cc: Likewise.