The x86 attributes such as ms_abi, stdcall, fastcall etc. alter the
function type, which means that functions with one of those attributes
do not match any of the partial specializations of std::is_function.

Rather than duplicating the list for every calling convention, use a
much simpler definition of std::is_function.

Also redefine __is_referenceable to not rely on partial specializations
for each type of referenceable function.

	PR libstdc++/91371
	* include/std/type_traits (is_function): Simplify definition. Remove
	partial specializations for function types.
	(__is_referenceable): Simplify definition.
	* testsuite/20_util/bind/91371.cc: New test.
	* testsuite/20_util/is_function/91371.cc: New test.
	* testsuite/20_util/is_function/value.cc: Check more pointer types.
	* testsuite/20_util/is_member_function_pointer/91371.cc: New test.
	* testsuite/20_util/is_object/91371.cc: New test.

From-SVN: r274756