`Complex.polar` accepts Complex values as arguments for the polar form as long
as the value of the complex has no imaginary part (ie it is 'real'). In
`f_complex_polar` this is handled by extracting the real part of the arguments.
However in the case `polar` is called with only a single argument, the absolute
value (abs), then the Complex is created without applying a check on the type
of abs, meaning it is possible to create a Complex where the real part is itself
an instance of a Complex. This change removes the short circuit for the single
argument case meaning the real part extraction is performed correctly
(by f_complex_polar).

Also adds an example to `spec/ruby/core/complex/polar_spec.rb` to check that
the real part of a complex argument is correctly extracted and used in the
resulting Complex real and imaginary parts.

Implement Complex#<=> so that it is usable as an argument when
calling <=> on objects of other classes (since #coerce will coerce
such numbers to Complex).  If the complex number has a zero imaginary
part, and the other argument is a real number (or complex number with
zero imaginary part), return -1, 0, or 1.  Otherwise, return nil,
indicating the objects are not comparable.

Fixes [Bug #15857]