cc-mode: extend regexp used by ‘c-or-c++-mode’

cc-mode: extend regexp used by ‘c-or-c++-mode’

* lisp/progmodes/cc-mode (c-or-c++-mode--regexp): Expand the regexp to
match some more C++-only constructs and recognise a few more standard
C++ header files.  Also make sure identifiers start with non-digit.
(c-or-c++-mode): Add ‘(interactive)’ declaration.

* test/lisp/progmodes/cc-mode-tests.el (c-or-c++-mode): Add test case
for the newly recognised constructs.