Reimplement the structural-equality checks used to determine whether

Reimplement the structural-equality checks used to determine whether
two types in different AST contexts are equivalent. Rather than
transforming the type from one context into the other context, we
perform a deep structural comparison of the types. This change
addresses a serious problem with recursive data types like

  struct ListNode {
    int value;
    struct ListNode *Next;
  } xList;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96278 91177308-0d34-0410-b5e6-96231b3b80d8