potentially misleading STYLE-WARNING for bogus IGNORE/IGNORABLE declarations

[Nikodemus Siivola]

 CL-USER> (let ((x 0))
            (let ((y x))
              (declare (ignore x))
              y))
 ; in: LAMBDA NIL
 ; (IGNORE X)
 ;
 ; caught STYLE-WARNING:
 ; declaring unknown variable X to be ignored
 ;
 ; compilation unit finished
 ; caught 1 STYLE-WARNING condition

Something along the lines of

  "declaring variable X, which is bound in outer scope, ignored"

...or anyways something that makes make it clear that X isn't unknown per-se, just that it isn't bound by the form where the declaration appears.

[Stas Boukarev (stassats)]

In 0191acbbad60897820741aa437d5172d78c35c18.