Fixed another regression introduced by r51113 caused by some refactoring

in Sema::CheckFunctionCall:

  http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20080512/005706.html
  
The bug was that the logic from the helper methods used by CheckFunctionCall
were being inverted (a subtle bug).  This would cause the parser to discard
any valid AST nodes involving several builtins (see patch).

This removes the last regression failure I'm seeing in the test suite: Analysis-Apple/NoReturn.

llvm-svn: 51168