bug#28439: suggestion: support case-independent xref-find-definitions prompt

[Winston]

Re: using completion-ignore-case to control both completions and
    xref-find-definitions 

Dmitry previously commented:
> We can do that without an extra variable, though, by making
> definitions search be controlled by completion-ignore-case.

   Unless you want to argue that xref-find-definitions is a
"completion", doing that would mean that completion-ignore-case has an
unexpected side-effect (namely, that it affects a function that is not a
"completion").  On programming aesthetic grounds, then, I'd argue that
xref-ignore-case (or whatever you want to call it) would be better.
 -WBE