libstdc++: Fix type-erasure in experimental::net::executor (PR 94203)

libstdc++: Fix type-erasure in experimental::net::executor (PR 94203)

The _Tgt and _TgtImpl types that implement type-erasure didn't agree on
the virtual interface, so failed as soon as they were instantiated. With
Clang they failed even sooner. The interface was also dependent on
whether RTTI was enabled or not.

This patch fixes the broken virtual functions and makes the type work
without RTTI, by using a pointer to a specialization of a function
template (similar to the approaches in std::function and std::any).

The changes to the virtual functions would be an ABI change, except that
the previous code didn't even compile if instantiated. This is
experimental TS material anyway.

Backport from mainline
2020-03-18  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/94203
	* include/experimental/executor (executor::executor(Executor)): Call
	make_shared directly instead of _M_create. Create _Tgt1 object.
	(executor::executor(allocator_arg_t, const ProtoAlloc&, Executor)):
	Call allocate_shared directly instead of _M_create. Create _Tgt2
	object.
	(executor::target_type): Add cast needed for new _Tgt interface.
	(executor::target): Define when RTTI is disabled. Use _Tgt::_M_func.
	(executor::_Tgt): Define the same interface whether RTTI is enabled or
	not.
	(executor::_Tgt::target_type, executor::_Tgt::target): Do not use
	std::type_info in the interface.
	(executor::_Tgt::_M_func): Add data member.
	(executor::_TgtImpl): Replace with _Tgt1 and _Tgt2 class templates.
	(executor::_Tgt1::_S_func): Define function to access target without
	depending on RTTI.
	(executor::_M_create): Remove.
	(operator==, operator!=): Simplify comparisons for executor.
	* include/experimental/socket (is_error_code_enum<socket_errc>):
	Define specialization before use.
	* testsuite/experimental/net/executor/1.cc: New test.