CacheMemory: add allocateVoid() that is == allocate() but no return value.

This function duplicates the functionality of allocate() exactly, except that it does not return
a return value.  In protocols where you just want to allocate a block
but do not want that block to be your implicitly passed cache_entry, use this function.
Otherwise, SLICC will complain if you do not consume the pointer returned by allocate(),
and if you do a dummy assignment Entry foo := cache.allocate(address), the C++
compiler will complain of an unused variable.  This is kind of a hack to get around
those issues, but suggestions welcome.

src/mem/protocol/RubySlicc_Types.sm

@@ -134,6 +134,7 @@ structure (CacheMemory, external = "yes") {
   bool cacheAvail(Address);
   Address cacheProbe(Address);
   AbstractCacheEntry allocate(Address, AbstractCacheEntry);
+  void allocateVoid(Address, AbstractCacheEntry);
   void deallocate(Address);
   AbstractCacheEntry lookup(Address);
   bool isTagPresent(Address);