Add a new AliassetTracker::remove method. Because we need to be able to remove

Add a new AliassetTracker::remove method.  Because we need to be able to remove
a pointer from an AliasSet, maintain the pointer values on a doubly linked
list instead of a singly linked list, to permit efficient removal from the
middle of the list.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10506 91177308-0d34-0410-b5e6-96231b3b80d8