[PATCH] Warn on suspicious use of absolute value function

[Marshall Clow]

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

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