c++/coroutines: only defer expanding co_{await,return,yield} if dependent [PR112341]

c++/coroutines: only defer expanding co_{await,return,yield} if dependent [PR112341]

By doing so, we can get diagnostics in template decls when we know we
can.  For instance, in the following:

  awaitable g();
  template<typename>
  task f()
  {
    co_await g();
    co_yield 1;
    co_return "foo";
  }

... the coroutine promise type in each statement is always
std::coroutine_handle<task>::promise_type, and all of the operands are
not type-dependent, so we can always compute the resulting types (and
expected types) of these expressions and statements.

Also, when we do not know the type of the CO_AWAIT_EXPR or
CO_YIELD_EXPR, we now return NULL_TREE as the type rather than
unknown_type_node.  This is more correct, since the type is not unknown,
it just isn't determined yet.  This also means we can remove the
CO_AWAIT_EXPR and CO_YIELD_EXPR special-cases from
type_dependent_expression_p.

PR c++/112341 - error: insufficient contextual information to determine type on co_await result in function template

gcc/cp/ChangeLog:

	PR c++/112341
	* coroutines.cc (struct coroutine_info): Also cache the
	traits type.
	(ensure_coro_initialized): New function.  Makes sure we have
	initialized the coroutine state successfully, or informs the
	caller should it fail to do so.  Extracted from
	coro_promise_type_found_p.
	(coro_get_traits_class): New function.  Gets the (cached)
	coroutine traits type for a given coroutine.  Extracted from
	coro_promise_type_found_p and refactored to cache the result.
	(coro_promise_type_found_p): Use the two functions above.
	(build_template_co_await_expr): New function.  Builds a
	CO_AWAIT_EXPR representing a CO_AWAIT_EXPR in a template
	declaration.
	(build_co_await): Use the above if processing_template_decl, and
	give it a proper type.
	(coro_dependent_p): New function.  Returns true iff its
	argument is a type-dependent expression OR the current functions
	traits class is type dependent.
	(finish_co_await_expr): Defer expansion only in the case
	coro_dependent_p returns true.
	(finish_co_yield_expr): Ditto.
	(finish_co_return_stmt): Ditto.
	* pt.cc (type_dependent_expression_p): Do not treat
	CO_AWAIT/CO_YIELD specially.

gcc/testsuite/ChangeLog:

	PR c++/112341
	* g++.dg/coroutines/pr112341-2.C: New test.
	* g++.dg/coroutines/pr112341-3.C: New test.
	* g++.dg/coroutines/torture/co-yield-03-tmpl-nondependent.C: New
	test.
	* g++.dg/coroutines/pr112341.C: New test.