[PATCH] Warn on suspicious use of absolute value function

[Richard Trieu]

  > Is this meant to be just for ‘abs’, or will it be generalized for all the calls in <math.h>?

  I haven't analyzed the use of the other functions in math.h to see how prevalent the problem with other functions are, but this warning could be generalized to other functions.

  > What about in C++ code, where there can be many functions named “abs” that take different arguments? What will this patch do in that case?
  > [ Ok, they’re named std::abs, but people hoist them into the global namespace _all the time_ ]

  It is also a common mistake to forget "std::", calling abs instead of std::abs when both are present, so a surprising bit of C++ code uses the wrong absolute value function.  I plan on extending this to provide some checking on std::abs.

http://llvm-reviews.chandlerc.com/D2224