Speed up project-find-regexp for simple regexps

Speed up project-find-regexp for simple regexps

* lisp/progmodes/xref.el (xref--regexp-syntax-dependent-p):
New function.
(xref--collect-matches): Use it.  Don't try to enable the
appropriate major mode and file-local variables if the regexp
does not depend on the buffer's syntax (bug#26710).
(xref--collect-matches-1): Don't syntax-propertize in that
case either.