coroutines: Implement n4849 changes to exception handling.

The standard now calls up a revised mechanism to handle exceptions
where exceptions thrown by the await_resume () method of the
initial suspend expression are considered in the same manner as
exceptions thrown by the user-authored function body.

This implements [dcl.fct.def.coroutine] / 5.3.

gcc/cp/ChangeLog:

2020-03-26  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (co_await_expander): If we are expanding the
	initial await expression, set a boolean flag to show that we
	have now reached the initial await_resume() method call.
	(expand_co_awaits): Handle the 'initial await resume called' flag.
	(build_actor_fn): Insert the initial await expression into the
	start of the user-authored function-body. Handle the 'initial await
	resume called' flag.
	(morph_fn_to_coro): Initialise the 'initial await resume called'
	flag.  Modify the unhandled exception catch clause to recognise
	exceptions that occur before the initial await_resume() and re-
	throw them.

gcc/testsuite/ChangeLog:

2020-03-26  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C: New test.

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