Disable -Wunique-enum for anonymous enums.

This is a large class of false positives where anonymous enums are used to
declare constants (see Clang's Diagnostics.h for example). A small number of
true positives could probably be found in this bucket by still warning if the
anonymous enum is used in a declarator (enum { ... } x;) but so far we don't
believe this to be a source of significant benefit so I haven't bothered to
preserve those cases.

General offline review/acknowledgment by rtrieu.

llvm-svn: 157713