libstdc++: Simplify std::__throw_bad_variant_access

libstdc++: Simplify std::__throw_bad_variant_access

This removes the overload of __throw_bad_variant_access that must be
called with a string literal. This avoids a potential source of
undefined behaviour if that function got misused. The other overload
that takes a bool parameter can be adjusted to take an integer index
selecting one of the four possible string literals to use, ensuring
that the std::bad_variant_access constructor is only called with those
literals.

Passing an index outside the range [0,3] is bogus, but will still select
a valid string literal and avoid undefined behaviour.

libstdc++-v3/ChangeLog:

	* include/std/variant (__throw_bad_variant_access(unsigned)):
	Define new function as inline friend, with namespace-scope
	declaration using noreturn attribute.
	(__throw_bad_variant_access(const char*)): Remove.
	(__throw_bad_variant_access(bool)): Remove.
	(visit, visit<R>): Adjust calls to __throw_bad_variant_access.

Reviewed-by: Patrick Palka <ppalka@redhat.com>