Allow xref to use other than current major-mode

[Stephen Leake]

I have the following use case:

I'm in a C buffer, asking "what elisp function will
semantic-parse-region dispatch to in this buffer?".

semantic-parse-region is a mode-local overloadable function; it
dispatches based on the current major mode. elisp--xref-find-definitions
doesn't currently show mode-local overrides; I'm working on adding that
capability.

I'd like to use xref-find-definitions to answer this question, so I
invoke it with C-u M-. 

That prompts me for an identifier. However, it uses the c-mode value of
xref-identifier-completion-table-function, so I can't complete to
semantic-parse-region.

In addition, xref--show-xrefs uses the c-mode value of
xref-find-function, so it won't find the elisp function.

To handle this, I propose changing both
xref-identifier-completion-table-function and xref-find-function to be
mode-local, instead of buffer-local.

Then, when current-prefix-arg is non-nil, xref--read-identifier can
prompt for the mode to use when completing the identifier, and
xref--show-xrefs can use that mode when calling xref-find-function.


There are similar use cases; from a LaTeX document buffer, looking at the
description of an Ada function, find the Ada code that defines that
function. Or from a text mode notes file, look for a definition in any
one of several languages.

-- 
-- Stephe