[emacs] Use c-mode-common-hook, derive from "gnu"
[emacs] Use c-mode-common-hook, derive from "gnu"

Make it clear that the "llvm.org" style is deriving from "gnu" style,
and use the c-mode-common-hook instead of c-mode-hook and c++-mode-hook.

Differential Revision: http://reviews.llvm.org/D7035

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@226861 91177308-0d34-0410-b5e6-96231b3b80d8

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