coroutines: diagnose usage of alloca in coroutines

coroutines: diagnose usage of alloca in coroutines

We do not support it currently, and the resulting memory can only be
used inside a single resumption, so best not confuse the user with it.

	PR c++/115858 - Incompatibility of coroutines and alloca()

gcc/ChangeLog:

	* coroutine-passes.cc (execute_early_expand_coro_ifns): Emit a
	sorry if a statement is an alloca call.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr115858.C: New test.