coroutines: Back out mandate for tail-calls at O < 2 [PR94359]

coroutines: Back out mandate for tail-calls at O < 2 [PR94359]

For symmetric transfers to work with C++20 coroutines, it is
currently necessary to tail call the callee coroutine from resume
method of the caller coroutine.  However there are several targets
which don't support an indirect tail call to an arbitrary callee.

Unfortunately, the target 'function_ok_for_sibcall' is not usable
from the front end in all cases.  While it is possible to add a new
hook to cover this circumstance, it is too late in the release
cycle to be sure of getting the setting correct for all targets.

So, this patch backs out the use of function_ok_for_sibcall () and
the mandate of CALL_EXPR_MUST_TAIL_CALL from the symmetric
transfer.

Targets that can make indirect tail calls to arbitrary callees will
still be able to make use of the symmetric transfer (without risking
overrunning the stack) for optimization levels >= 2.

The draft standard does not mandate unlimited symmetric transfers,
so removing this is a QOI issue (albeit an important one) rather
than a correctness one.

The test is moved and adjusted so that it can be opted into by any
target that supports the necessary tailcall.

gcc/cp/ChangeLog:

2020-04-16  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94359
	* coroutines.cc (build_actor_fn): Back out use of
	targetm.function_ok_for_sibcall.  Do not mark the resume
	call as CALL_EXPR_MUST_TAIL_CALL.

gcc/testsuite/ChangeLog:

2020-04-16  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94359
	* g++.dg/coroutines/torture/symmetric-transfer-00-basic.C: Move..
	* g++.dg/coroutines/symmetric-transfer-00-basic.C: ..here and
	adjust to run at O2 for targets supporting the necessary tail
	call.