Add -Wduplicate-enum warning. Clang will emit this warning when an implicitly

Add -Wduplicate-enum warning.  Clang will emit this warning when an implicitly
initiated enum constant has the same value as another enum constant.

For instance:
enum test { A, B, C = -1, D, E = 1 };
Clang will warn that:
 A and D both have value 0
 B and E both have value 1

A few exceptions are made to keep the noise down.  Enum constants which are
initialized to another enum constant, or an enum constant plus or minus 1 will
not trigger this warning.  Also, anonymous enums are not checked.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162938 91177308-0d34-0410-b5e6-96231b3b80d8