coroutines, ensure placeholder var is properly declared.

In cases that we need to extended the lifetime of a temporary captured
by reference, we make a replacement var for the temporary.  This will
be then used to define a coroutine frame entry (so that the var created
is elided by a later phase).  However, we should ensure that the var
is correctly declared anyway.

gcc/cp/ChangeLog:

2020-04-07  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (maybe_promote_captured_temps): Ensure that
	reference capture placeholder vars are properly declared.

gcc/cp/ChangeLog

+2020-04-07  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* coroutines.cc (maybe_promote_captured_temps): Ensure that
+	reference capture placeholder vars are properly declared.
+
 2020-04-07  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/90996

gcc/cp/coroutines.cc

@@ -2832,7 +2832,9 @@ maybe_promote_captured_temps (tree *stmt, void *d)
 	    sloc = DECL_SOURCE_LOCATION (orig_temp);
 	  DECL_SOURCE_LOCATION (newvar) = sloc;
 	  DECL_CHAIN (newvar) = varlist;
-	  varlist = newvar;
+	  varlist = newvar; /* Chain it onto the list for the bind expr.  */
+	  /* Declare and initialze it in the new bind scope.  */
+	  add_decl_expr (newvar);
 	  tree stmt
 	    = build2_loc (sloc, INIT_EXPR, var_type, newvar, to_replace);
 	  stmt = coro_build_cvt_void_expr_stmt (stmt, sloc);