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

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.