Ada, treelang needs to be converted to --enable-mapped-location

[Richard Kenner]

    It can certainly be made to work.  I am not in favor of this change,
    though, as it requires adding call-back hooks, which are generally to
    be avoided, and which in this case are unnecessary, even if they make
    it easier in the short term.

Hook, singular.  It means that there's a language-dependent version of
expand_location, that's all.  The advantage is that it requires *no other
change* anywhere except to make Sloc_to_locus a nop in the Ada front end.