As reported in the PR, per [dcl.fct.def.coroutine]/4 we should
be passing a reference to the object to the promise parameter
preview, and we are currently passing a pointer (this).  Amend to
pass the reference.

gcc/cp/ChangeLog:

2020-04-22  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94682
	* coroutines.cc (struct param_info): Add a field to note that
	the param is 'this'.
	(morph_fn_to_coro): Convert this to a reference before using it
	in the promise parameter preview.

gcc/testsuite/ChangeLog:

2020-04-22  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94682
	* g++.dg/coroutines/pr94682-preview-this.C: New test.