Drop warning emitted for comparison of constants of different domains

- Such values are simply compared unequal, even if their underlying
  values are equal.  The warning would be impractical for some of the
  uses, e.g. it migth be legitimate to compare the values when
  sorting.  And it is of course fully OK to just ask whether any
  random value happens to be DW_AT_name and just get a false if it's,
  say, DW_TAG_subprogram instead.
Allow comparisons between differently-typed values

- Reasoning is similar to previous commit.  It should be possible to
  do e.g. this::

	[some expression @AT_something (== "blah")]

  ... without getting errors just because one of the attributes
  happens to not be a string.