Some queries could not be made diff-informed because they select an entity whose location is not a Location. Specifically this is true for the RegExpTerms inside a regular expression.

This PR addresses this by adding the ability to filter based on a location containing the final selected location. We then filter based on the location of the entire RegExp literal, which is valid because it encloses the location of all its terms.

Unfortunately the Ruby query still can't be made diff-informed because the locations of its RegExpTerms aren't correct when the regexp is parsed from a string arising from constant folding. But it fixes it for Python and Java.

Initially in this PR the approximate location filtering was opt-in via a separate API, but after discussing with @jbj it is then made the default in a later commit.