coros: mark .CO_YIELD as LEAF [PR106973]

We rely on .CO_YIELD calls being followed by an assignment (optionally)
and then a switch/if in the same basic block.  This implies that a
.CO_YIELD can never end a block.  However, since a call to .CO_YIELD is
still a call, if the function containing it calls setjmp, GCC thinks
that the .CO_YIELD can introduce abnormal control flow, and generates an
edge for the call.

We know this is not the case; .CO_YIELD calls get removed quite early on
and have no effect, and result in no other calls, so .CO_YIELD can be
considered a leaf function, preventing generating an edge when calling
it.

PR c++/106973 - coroutine generator and setjmp

	PR c++/106973

gcc/ChangeLog:

	* internal-fn.def (CO_YIELD): Mark as ECF_LEAF.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr106973.C: New test.