Add new -Wunique-enum which will warn on enums which all elements have the

Add new -Wunique-enum which will warn on enums which all elements have the
same value and were initialized with literals.  Clang will warn on code like
this:

enum A {
  FIRST = 1,
  SECOND = 1
};

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157666 91177308-0d34-0410-b5e6-96231b3b80d8