coroutines: Support for debugging implementation state.

coroutines: Support for debugging implementation state.

Some of the state that is associated with the implementation
is of interest to a user debugging a coroutine.  In particular
items such as the suspend point, promise object, and current
suspend point.

These variables live in the coroutine frame, but we can inject
proxies for them into the outermost bind expression of the
coroutine.  Such variables are automatically moved into the
coroutine frame (if they need to persist across a suspend
expression).  PLacing the proxies thus allows the user to
inspect them by name in the debugger.

To implement this, we ensure that (at the outermost scope) the
frame entries are not mangled (coroutine frame variables are
usually mangled with scope nesting information so that they do
not clash).  We can safely avoid doing this for the outermost
scope so that we can map frame entries directly to the variables.

This is partial contribution to debug support (PR 99215).

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

	* coroutines.cc (register_local_var_uses): Do not mangle
	frame entries for the outermost scope.  Record the outer
	scope as nesting depth 0.