Literals that are used to represent the bits of certain floating point
numbers (e.g. VTKM_NAN_BITS_64) are placed into unsigned integers before
converted to floating points. We ran into an example where the compiler
complained that a literal (specifically VTKM_NEG_INF_BITS_64) was
declared signed and was negative but then placed in an unsigned (64-bit)
integer. This should fix the problem by making the literal itself
unsigned.