libstdc++: Constrain net::executor constructors

The TS says the arguments to these constructors shall meet the Executor
requirements, so it's undefined if they don't. Constraining on a subset
of those requirements won't affect valid cases, but prevents the
majority of invalid cases from trying to instantiate the constructor.

This prevents the non-explicit executor(Executor) constructor being a
candidate anywhere that a net::executor could be constructed e.g.
comparing ip::tcp::v4() == ip::udp::v4() would try to convert both
operands to executor using that constructor, then compare then using
operator==(const executor&, const executor&).

libstdc++-v3/ChangeLog:

	* include/experimental/executor (executor): Constrain template
	constructors.