Implement -Wenum-compare, which warns when comparing two enums of

different types. We omit the warning when the enum types are anonymous.
Unlike GCC, this warning does not distinguish between C++ and C/ObjC for
controling whether it is on by default, it is always on by default.

Original patch contributed by Richard Trieu (@ Google), I fixed some
style issues, and cleaned it up for submission.

llvm-svn: 125739