[PATCH] Fix to PR15537 - assertion failure when comparing atomic unsigned int to int.

[Serge Pavlov]

  Comments to Richard's notes:
  1) This change affects only comparison operations,
  2) It looks like UsualArithmeticConversions is not appropriate place for this conversion. That function implements usual arithmetic conversions applicable to arithmetic types as described by C11 6.3.1.8p1 . According to C11 6.2.5p20 atomic types aren't arithmetic but derived. Replacing the atomic type by its basic type is allowed by lvalue conversion described by C11 6.3.2.1p2.

http://llvm-reviews.chandlerc.com/D599