c++, coroutines: Make sure our temporaries are in a bind expr [PR105287]

There are a few cases where we can generate a temporary that does not need
to be added to the coroutine frame (i.e. these are genuinely ephemeral).  The
intent was that unnamed temporaries should not be 'promoted' to coroutine
frame entries.  However there was a thinko and these were not actually ever
added to the bind expressions being generated for the expanded awaits.  This
meant that they were showing in the global namspace, leading to an empty
DECL_CONTEXT and the ICE reported.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

	PR c++/105287

gcc/cp/ChangeLog:

	* coroutines.cc (maybe_promote_temps): Ensure generated temporaries
	are added to the bind expr.
	(add_var_to_bind): Fix local var naming to use portable punctuation.
	(register_local_var_uses): Do not add synthetic names to unnamed
	temporaries.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr105287.C: New test.