Fix undefined behavior when translating define_variable forms

Fix undefined behavior when translating define_variable forms

When translating a define_variable form using translate (but not
using, e.g., translate_file or compfile), a DECLARE expression was
being evaluated.  In CL it is undefined what happens when evaluating
such an expression, and this would cause a lisp error under some
lisp implementations.

Now we use DECLAIM instead, since this is not emitted in the
special places where DECLARE expressions are allowed.

No problems with the test suite or share test suite.  A new test
has been added to rtest_translator.