libstdc++: Remove inheritance from std::coroutine_handle<> [LWG 3460]

libstdc++: Remove inheritance from std::coroutine_handle<> [LWG 3460]

This removes the coroutine_handle<> base class from the primary template
and the noop_coroutine_promise explicit specialization. To preserve the
API various members are added, as they are no longer inherited from the
base class.

I've also tweaked some indentation and formatting, and replaced
subclause numbers from the standard with stable names like
[coroutine.handle.con].

libstdc++-v3/ChangeLog:

	* include/std/coroutine (coroutine_handle<_Promise>): Remove
	base class. Add constructors, conversions, accessors etc. as
	proposed for LWG 3460.
	(coroutine_handle<noop_coroutine_promise>): Likewise.
	* testsuite/18_support/coroutines/lwg3460.cc: New test.