Fix PR c++/66590: incorrect warning "reaches end of non-void function" for switch

Fix PR c++/66590: incorrect warning "reaches end of non-void function" for switch

So the problem here is there is code in the C++ front-end not to add a
break statement (to the IR) if the previous block does not fall through.
The problem is the code which does the check to see if the block
may fallthrough does not check a CLEANUP_STMT; it assumes it is always
fall through.  Anyways this adds the code for the case of a CLEANUP_STMT
that is only for !CLEANUP_EH_ONLY (the try/finally case).

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/cp/ChangeLog:

	PR c++/66590
	* cp-objcp-common.c (cxx_block_may_fallthru): Handle
	CLEANUP_STMT for the case which will be try/finally.

gcc/testsuite/ChangeLog:

	PR c++/66590
	* g++.dg/warn/Wreturn-5.C: New test.