c++/coroutines: correct passing *this to promise type [PR104981]

c++/coroutines: correct passing *this to promise type [PR104981]

When passing *this to the promise type ctor (or to its operator new)
(as per [dcl.fct.def.coroutine]/4), we add an explicit cast to lvalue
reference.  But this is unnecessary since *this is already always an
lvalue.  And doing so means we need to call convert_from_reference
afterward to lower the reference expression to an implicit dereference,
which we're currently neglecting to do and which causes overload
resolution to get confused when computing argument conversions.

So this patch removes this unneeded reference cast when passing *this
to the promise ctor, and removes both the cast and implicit deref when
passing *this to operator new, for consistency.  While we're here, use
cp_build_fold_indirect_ref instead of directly building INDIRECT_REF.

	PR c++/104981
	PR c++/115550

gcc/cp/ChangeLog:

	* coroutines.cc (morph_fn_to_coro): Remove unneeded calls
	to convert_to_reference and convert_from_reference when
	passing *this.  Use cp_build_fold_indirect_ref instead
	of directly building INDIRECT_REF.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr104981-preview-this.C: New test.
	* g++.dg/coroutines/pr115550-preview-this.C: New test.

Reviewed-by: Iain Sandoe <iain@sandoe.co.uk>
Reviewed-by: Jason Merrill <jason@redhat.com>