CC Mode: fix minor errors in the fontification of enums.

CC Mode: fix minor errors in the fontification of enums.

1: Fix the typing of a space on the { line in an enum
specification which is the first item in a buffer.  This
defontified the enum's identifiers.
2: Fix the typing of a space on the line of the enum's last
identifier.  This defontified that identifier for
jit-lock-context-time.

* lisp/progmodes/cc-engine.el (c-forward-declarator): Cease
rejecting the identifier if there are no further tokens before
LIMIT.

* lisp/progmodes/cc-mode.el (c-fl-decl-start): Accept a
backward search as succeeding if it hits point-min.

(cherry picked from commit 086cb1b590251ca353fec5a841ea882cdf9cbcb2)