Clean up some Sema checking code. NFC

- Rename CheckMinZero to CheckMaxUnsignedZero to reflect its actual purpose.
- Remove unused parameters from CheckAbsoluteValueFunction and
  CheckMaxUnsignedZero functions.
- Refactor the function name check so both functions can use the same one.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288756 91177308-0d34-0410-b5e6-96231b3b80d8