c++: fix broken conversion in coroutines

c++: fix broken conversion in coroutines

You can't use CONVERT_EXPR to convert between two class types.

VIEW_CONVERT_EXPR takes liberties with the C++ type system, but is probably
safe in this context.  Let's also only use it when the type isn't already
what we want.

gcc/cp/ChangeLog:

	* coroutines.cc (expand_one_await_expression): Change conversion
	to VIEW_CONVERT_EXPR.
	* cp-gimplify.cc (cp_genericize_r) [CONVERT_EXPR]: Add assert.