c++, coroutines: Fix block nests when the function has no top-level bind.

c++, coroutines: Fix block nests when the function has no top-level bind.

When the function contains no local vars and also no nested scopes, there
is no top-level bind expression.  Because the rewritten coroutine body will
require both local vars and contain nested scopes, we add a bind expression
to such functions.  When this was done the necessary scope blocks were
omitted which leads to disconnected function content.

Fixed by adding a new block to the added bind expression.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Ensure that added
	bind expressions have scope blocks.

(cherry picked from commit a8d7631d333c22e38a067d32d11fd2b60cf1d960)