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

[chilledheart]

Summary

It will be quite convenient to use if fallback style support is added,
so that clang-format.py will search and use existing .clang-format file
if there is any and fallback to the specific format style if not.
It should cover the projects which don't have .clang-format files
in their source.

And option fallback-style is available in clang 3.5 or later, it is safe to use.

[djasper]

Sorry, this somehow got lost. Submitted as r235080.

tools/clang-format/clang-format.py

 if vim.eval('exists("g:clang_format_path")') == "1":
   binary = vim.eval('g:clang_format_path')
 
 # Change this to format according to other formatting styles. See the output of
 # 'clang-format --help' for a list of supported styles. The default looks for
 # 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.
   buf = vim.current.buffer
   text = '\n'.join(buf)
 
   # Determine range to format.
   lines = '%s:%s' % (vim.current.range.start + 1, vim.current.range.end + 1)
 
   # Determine the cursor position.
   cursor = int(vim.eval('line2byte(line("."))+col(".")')) - 2
   if cursor < 0:
     print 'Couldn\'t determine cursor position. Is your file empty?'
     return
 
   # Avoid flashing an ugly, ugly cmd prompt on Windows when invoking clang-format.
   startupinfo = None
   if sys.platform.startswith('win32'):
     startupinfo = subprocess.STARTUPINFO()
     startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
     startupinfo.wShowWindow = subprocess.SW_HIDE
 
   # 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,
                        stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                        stdin=subprocess.PIPE, startupinfo=startupinfo)
   stdout, stderr = p.communicate(input=text)
 
   # If successful, replace buffer contents.