clang-format: add an option for fallback style in vimrc

With this patch, clang-format.py will search and use existing .clang-format
file if there is one and fallback to the specific format style if
not. It should cover the projects which don't have .clang-format
files in their source. As the option fallback-style is available in
clang 3.5 or later, it is safe to use.

Patch by "Chilledheart" (http://reviews.llvm.org/D8489).

llvm-svn: 235080