Make electric-pair-mode, delete-selection-mode and CC Mode cooperate.
Fixes debbugs#21275.
In Emacs >= 25, let electric-pair-mode take precedence over
delete-selection-mode.
delsel.el (delete-selection-uses-region-p): New function, previously a
lambda expression in a property value for `self-insert-command'.
(top-level) Set the `delete-selection' property of `self-insert-command'
to `delete-selection-uses-region-p'.
progmodes/cc-cmds.el (top-level): Give the `delete-selection' property
for c-electric-\(brace\|paren\) the value `delete-selection-uses-region-p'
when the latter function exists.