c++: top level bind when rewriting coroutines [PR106188]

c++: top level bind when rewriting coroutines [PR106188]

In the edge case of a coroutine not containing any locals, the ifcd/switch
temporaries would get added to the coroutine frame, corrupting its
layout. To prevent this, we can make sure there is always a BIND_EXPR at
the top of the function body, and thus, always a place for our new
temporaries to go without interfering with the coroutine frame.

PR c++/106188 - Incorrect frame layout after transforming conditional statement without top-level bind expression
PR c++/106713 - if (co_await ...) crashes with a jump to ud2

	PR c++/106188
	PR c++/106713

gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Ensure we have a
	BIND_EXPR wrapping the function body.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr106188.C: New test.