[PATCH] Warn on suspicious use of absolute value function

[Richard Trieu]

This is a new warning to detect improper uses of the absolute value function.  Three new warnings are introduced with this patch.

1) Attempting to take the absolute value of an unsigned integer.  Includes fix-it to remove the absolute value function call.
2) Using an absolute value function that is too small.  For instance, using abs(int) on a long would give a warning and a fix-it to use labs.
3) Using the wrong type of absolute value (integer, floating point, complex) for the argument given.  Includes a fix-it to the proper absolute value type.

I have already run this over a large code base and found hundreds of questionable usages of absolute value.

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

Files:
  test/Sema/warn-absolute-value.c
  include/clang/Basic/DiagnosticSemaKinds.td
  include/clang/Basic/DiagnosticGroups.td
  include/clang/Sema/Sema.h
  include/clang/AST/Decl.h
  lib/Sema/SemaChecking.cpp
  lib/AST/Decl.cpp
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