coroutines: Update to n4849 allocation/deallocation.

This updates the coroutine frame allocation and deallocation usage to
match n4849.

[dcl.fct.def.coroutine] /9, /10, /12.

9 An implementation may need to allocate additional storage for a coroutine.
This storage is known as the coroutine state and is obtained by calling a
non-array allocation function. The allocation function’s name is looked up
in the scope of the promise type. If this lookup fails, the allocation
function’s name is looked up in the global scope. If the lookup finds an
allocation function in the scope of the promise type, overload resolution
is performed on a function call created by assembling an argument list.
The first argument is the amount of space requested, and has type
std::size_t. The lvalues p1 . . . pn are the succeeding [user's function]
arguments. If no viable function is found, overload resolution is performed
again on a function call created by passing just the amount of space required
as an argument of type std::size_t.

10 The unqualified-id get_return_object_on_allocation_failure is looked up in
the scope of the promise type by class member access lookup. If any
declarations are found, then the result of a call to an allocation function
used to obtain storage for the coroutine state is assumed to return nullptr
if it fails to obtain storage, and if a global allocation function is
selected, the ::operator new(size_t, nothrow_t) form is used. The allocation
function used in this case shall have a non-throwing noexcept-specification.
If the allocation function returns nullptr, the coroutine returns control to
the caller of the coroutine and the return value is obtained by a call to
T::get_return_object_on_allocation_failure(), where T is the promise type.

12 The deallocation function’s name is looked up in the scope of the promise
type. If this lookup fails, the deallocation function’s name is looked up in
the global scope. If deallocation function lookup finds both a usual
deallocation function with only a pointer parameter and a usual deallocation
function with both a pointer parameter and a size parameter, then the
selected deallocation function shall be the one with two parameters.
Otherwise, the selected deallocation function shall be the function with one
parameter. If no usual deallocation function is found, the program is ill-
formed. The selected deallocation function shall be called with the address
of the block of storage to be reclaimed as its first argument. If a
deallocation function with a parameter of type std::size_t is used, the size
of the block is passed as the corresponding argument.

gcc/cp/ChangeLog:

2020-02-12 Iain Sandoe <iain@sandoe.co.uk>

	* coroutines.cc (build_actor_fn): Implement deallocation function
	selection per n4849, dcl.fct.def.coroutine bullet 12.
	(morph_fn_to_coro): Implement allocation function selection per
	n4849, dcl.fct.def.coroutine bullets 9 and 10.

2020-02-12 Iain Sandoe <iain@sandoe.co.uk>

	* g++.dg/coroutines/coro1-allocators.h: New.
	* g++.dg/coroutines/coro-bad-alloc-00-bad-op-new.C: New test.
	* g++.dg/coroutines/coro-bad-alloc-01-bad-op-del.C: New test.
	* g++.dg/coroutines/coro-bad-alloc-02-no-op-new-nt.C: New test.
	* g++.dg/coroutines/torture/alloc-00-gro-on-alloc-fail.C: Use new
	coro1-allocators.h header.
	* g++.dg/coroutines/torture/alloc-01-overload-newdel.C: Likewise.
	* g++.dg/coroutines/torture/alloc-02-fail-new-grooaf-check.C: New.
	* g++.dg/coroutines/torture/alloc-03-overload-new-1.C: New test.
	* g++.dg/coroutines/torture/alloc-04-overload-del-use-two-args.C:New.

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