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).

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@235080 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
diff --git a/clang-format.py b/clang-format.py
index a79205a..b07160e 100644
--- a/clang-format.py
+++ b/clang-format.py
@@ -34,6 +34,8 @@
 # a '.clang-format' or '_clang-format' file to indicate the style that should be
 # used.
 style = 'file'
+if vim.eval('exists("g:clang_format_fallback_style")') == "1":
+  fallback_style = vim.eval('g:clang_format_fallback_style')
 
 def main():
   # Get the current text.
@@ -58,6 +60,8 @@
 
   # Call formatter.
   command = [binary, '-lines', lines, '-style', style, '-cursor', str(cursor)]
+  if fallback_style:
+    command.extend(['-fallback-style', fallback_style])
   if vim.current.buffer.name:
     command.extend(['-assume-filename', vim.current.buffer.name])
   p = subprocess.Popen(command,