Clang should warn when `<` or `>` is used instead of shift operators in enum initializers

[ahatanak]

clang should diagnose cases where comparison operators (< or >) are used instead of shift operators (<< or >>) in enumerator initializers. This is a common mistake that often occurs when defining bit flags or option enums.

For example:

enum E {
  kOptionOne    = 1ull << 0, // correct
  kOptionTwo    = 1ull < 1,  // oops!
  kOptionThree = 1ull < 2,  // oops!
};

Although these are valid C/C++ expressions, they are very likely to be errors in this context.

[GrumpyPigSkin]

I would be happy to take a look at this :)