When called with prefix argument, ask for file patterns to search as well

When called with prefix argument, ask for file patterns to search as well

* lisp/progmodes/xref.el (xref-find-regexp): When called with
prefix argument, ask for file patterns to search as well.  When
prompting for the directory, require an existing one.
(xref-collect-matches): Add a new argument, FILES.  Use it in the
above function.