add assertion to catch AccessSet usage errors

I ran into this while implementing
async actor inits; where I was emitting
an actor hop in the middle of
an access region, which caused the
access set to be unexpectedly empty.

Author: kavon

stdlib/public/runtime/Exclusivity.cpp

@@ -202,6 +202,7 @@ class AccessSet {
   }
 
   void remove(Access *access) {
+    assert(Head && "removal from empty AccessSet");
     auto cur = Head;
     // Fast path: stack discipline.
     if (cur == access) {