Perform xref searches without visiting unopened files

Perform xref searches without visiting unopened files

* lisp/progmodes/xref.el (xref-collect-references): Instead of
calling `semantic-symref-find-references-by-name', use
`semantic-symref-instantiate' and `semantic-symref-perform-search'
directly.  Ask for `line-and-text' results (bug#23223).
(xref-collect-matches): Include the line text in the "hit"
structure.
(xref--convert-hits): New function, split off from
`xref-collect-references' and `xref-collect-matches', to convert
"hits" to xref instance list.  Create a temporary buffer here, to
use it for post-processing all hit lines.
(xref--collect-matches): Use a different approach for non-visited
files.  Insert the line text into the temp buffer, apply the
file's major mode the best we can without reading its whole
contents, syntax-propertize, and search in the result.
(xref--collect-matches-1): Extract, to handle the common logic
between two cases.
(xref--find-buffer-visiting): New function, a wrapper around
`find-buffer-visiting' to amortize its cost.

* lisp/cedet/semantic/symref/idutils.el
(semantic-symref-idutils--line-re): New constant.
(semantic-symref-parse-tool-output-one-line): Support result type
`line-and-text'.

* lisp/cedet/semantic/symref/grep.el
(semantic-symref-grep--line-re)
(semantic-symref-parse-tool-output-one-line): Same.

* lisp/cedet/semantic/symref/cscope.el
(semantic-symref-cscope--line-re)
(semantic-symref-parse-tool-output-one-line): Same.

* lisp/cedet/semantic/symref/global.el
(semantic-symref-global--line-re)
(semantic-symref-parse-tool-output-one-line): Same.