Replace faulty non-matching regexp "\\<\\>" with "a\\`"

Replace faulty non-matching regexp "\\<\\>" with "a\\`"

The regexp "\\<\\>", which is supposed never to match, actually matches, for
instance, where a Chinese character is directly followed by an ASCII letter.
So, replace it with "a\\`".

* lisp/progmodes/cc-defs.el (cc-fix, c-make-keywords-re)
* lisp/progmodes/cc-engine.el (c-beginning-of-statement-1)
(c-forward-<>-arglist-recur, c-forward-decl-or-cast-1)
(c-looking-at-decl-block)
* lisp/progmodes/cc-langs.el (c-assignment-op-regexp)
(c-block-comment-ender-regexp, c-block-comment-start-regexp)
(c-line-comment-start-regexp, c-doc-comment-start-regexp)
(c-decl-start-colon-kwd-re, c-type-decl-prefix-key)
(c-type-decl-operator-prefix-key, c-pre-id-bracelist-key)
(c-enum-clause-introduction-re, c-nonlabel-token-2-key)
* lisp/progmodes/cc-vars.el (c-noise-macro-with-parens-name-re)
(c-make-noise-macro-regexps):
Replace "\\<\\>" by "a\\`".